Research
 Computational Methodology <img src="http://www.weizmann.ac.il/materials/Leeor/sites/materials.Leeor/files/research_subject1.png" width="183" height="115" alt="" />
 Properties of Materials <img src="http://www.weizmann.ac.il/materials/Leeor/sites/materials.Leeor/files/research_subject2.png" width="183" height="115" alt="" />
 Formalism <img src="http://www.weizmann.ac.il/materials/Leeor/sites/materials.Leeor/files/research_subject3.png" width="182" height="115" alt="" />
Adventures in (Real) Space
Some twenty years ago, Chelikowsky, Saad, and coworkers suggested solving the KohnSham equations of density functional theory using a highorder finite difference approach on a realspace grid. For much of that time, our group has been a major partner in developing and applying this approach. It has now blossomed into a mature, massively parallel software suite, which we call PARSEC  the pseudopotential algorithm for realspace electronic structure calculations. This approach has many advantages. First and foremost, it produces Hamiltonian matrices that are very sparse. Therefore, the Hamiltonian is never computed or stored explicitly, but rather only its action on a wave function vector is computed. As a direct consequence, massively parallel calculations scale extremely well. Put in simpler language, when performing a parallel computation on a number of processors (from several to many thousands and more), for a large problem the computation time will decrease almost linearly with the increase in processor number. This, together with advances in diagonalization algorithms, allows us to attack problems with thousands of electrons.
The approach has other advantages as well:
 With respect to localized basis sets, it provides an approach without an explicit basis so that convergence is trivial (just decrease the grid spacing), and all those pesky basis set issues are avoided. This also means that no recurring basis setup and no spurious forces are associated with atom movement and that localized and delocalized electrons are treated on the same footing.
 With respect to planewave approaches, we can treat periodic and nonperiodic systems equally. This means that "supercells", which introduce spurious periodicity, are not employed. Consequently, problems "inherited" from the supercell, e.g., difficulties with treating monopolar or dipolar systems (e.g, in the study of charged defects or polar surfaces, respectively) are not encountered.
 A realspace grid is also a natural arena for solving the optimized effective potential equation, and is therefore an important tool in our studies of orbitaldependent functionals.
 The code is wellstructured and physically transparent. This makes the implementation of new ideas (relatively) easy!
 Last but not at all least, the approach serves as a basis for allelectron realspace calculations. In recent years, we have developed two such codes:
DARSEC – the diatomic algorithm for realspace electronic structure calculations: This program is a “cousin” of PARSEC, which employs a prolatespheroidal grid to allow for allelectron solutions of atoms and diatomic molecules.
CARMA – the concurrent allelectron realspace multigrid algorithm: This program uses a locallyrefined multigrid approach to obtain an allelectron solution for an arbitrary molecular system.
Some of our recent articles in this research direction are:
An overview of the PARSEC approach:
 L. Kronik, A. Makmal, M. Tiago, M. M. G. Alemany, X. Huang, Y. Saad, and J. R. Chelikowsky, "PARSEC  the pseudopotential algorithm for realspace electronic structure calculations: recent advances and novel applications to nanostructures", Phys. Stat. Solidi. (b) (Feature Article) 243, 1063 (2006).
Generalizations to periodicity, spinorbit coupling, noncollinear magnetism, and transport:
 D. Naveh, L. Kronik, M. L. Tiago, and J. R. Chelikowsky, "RealSpace Pseudopotential method for SpinOrbit Coupling within Density Functional Theory", Phys. Rev. B 76, 153407 (2007).
 A. Natan, A. Benjamini, D. Naveh, L. Kronik, M. L. Tiago, S. P. Beckman, and J. R. Chelikowsky, "Realspace pseudopotential method for first principles calculations of general periodic and partially periodic systems", Phys. Rev. B 78, 075109 (2008).
 D. Naveh and L. Kronik, "RealSpace Pseudopotential method for Noncollinear Magnetism within Density Functional Theory", Solid State Comm. 149, 177 (2009).
 B. Feldman, T. Seideman, O. Hod, L. Kronik, “A realspace method for highly parallelizable electronic transport calculations”, Phys. Rev. B 90, 035445 (2014).
Some recent applications of PARSEC:
 E. Kraisler and L. Kronik, “Fundamental gaps with approximate density functionals: the derivative discontinuity revealed from ensemble considerations”, J. Chem. Phys. 140, 18A540 (2014).
 R. Viswanatha, D. Naveh, J. R. Chelikowsky, L. Kronik and D. D. Sarma, “Magnetic properties of Fe/Cu codoped ZnO Nanocrystals”, J. Phys. Chem. Lett. 3, 2009 (2012).
 F. Rissner, A. Natan, D. A. Egger, O. T. Hofmann. L. Kronik and E. Zojer, “Dimensionality effects in the electronic structure of organic semiconductors consisting of polar repeat units”, Org. Electr. 13, 3165 (2012).
 F. Rissner, D. A. Egger, A. Natan, T. Körzdörfer, S. Kümmel, L. Kronik, and E. Zojer, “Collectively Induced QuantumConfined Stark Effect in Monolayers of Molecules Consisting of Polar Repeating Units”, J. Am. Chem. Soc. 133, 18634 (2011).
An overview of the DARSEC approach:
 A. Makmal, S. Kümmel, and L. Kronik, "Fully numerical allelectron solutions of the optimized effective potential equation for diatomic molecules", J. Chem. Theo. Comp. 5, 1731 (2009).
Some recent applications of DARSEC:
 E. Kraisler, T. Schmidt, S. Kümmel, and L. Kronik, “Effect of ensemble generalization on the highestoccupied KohnSham eigenvalue”, J. Chem. Phys. 143, 104105 (2015).
 T. Schmidt, E. Kraisler, L. Kronik, S. Kümmel, “Oneelectron selfinteraction and the asymptotics of the KohnSham potential: an impaired relation”, Phys. Chem. Chem. Phys. (Perspectives Article) 16, 14357 (2014).
 T. Schmidt, E. Kraisler, A. Makmal, L. Kronik, S. Kümmel, “A selfinteractionfree local hybrid functional: accurate binding energies visàvis accurate ionization potentials from KohnSham eigenvalues”, J. Chem. Phys. (Special issue on Density Functional Theory) 140, 18A510 (2014).
 E. Kraisler and L. Kronik, “Piecewise linearity of approximate density functionals revisited: Implications for frontier orbital energies”, Phys. Rev. Lett. 110, 126403 (2013).
 A. Makmal, S. Kümmel, and L. Kronik, "Dissociation of diatomic molecules and the exactexchange KohnSham potential: the case of LiF", Phys. Rev. A 83, 062512 (2011).
An overview of the CARMA approach:
 O. Cohen, L. Kronik, and A. Brandt, “Locally Refined Multigrid Solution of the AllElectron KohnSham Equation”, J. Chem. Theo. Comp. 9, 4744 (2013).
Organic and Molecular Electronics
Organic−inorganic hybrids
Supramolecular materials
Biogenic Materials
Organic and Molecular Electronics
Organic electronic materials are fascinating from both the basic and the applied science points of view, and bridge basic science and applied research, as well as physics and chemistry. Technologically, the electronic and optical properties of organic matter, including its interface with inorganic matter, are of tremendous importance for several emerging and nascent technologies. Chief among them is organic electronics, where electronic and optoelectronic devices (e.g., transistors, lightemitting diodes, and photovoltaic cells) rely on conjugated organic materials with semiconducting properties, rather than on traditional inorganic semiconductors. A related example is that of molecular electronics, where current conduction, rectification, information processing, and storage take place within a single molecule, and where the connection to inorganic leads is part and parcel of the device.
One obvious role of theory is to uncover the novel mechanisms for charge generation, transport, and transfer that allow the abovedescribed devices to function. But even more interesting, from the theoretical point of view, is that organic electronic materials and organic/inorganic interfaces often possess a range of surprising electronic properties, whose identification and explanation is a significant challenge for both experiment and theory. To a large extent, this is because understanding these properties forces us to bridge two different ``world views''  that of molecular orbital theory, which underlies much of organic chemistry, and that of delocalized electron waves, which underlies much of solidstate physics. One often encounters phenomena that are not welldescribed by either of the limiting textbook descriptions, and more elaborate theories need to be constructed.
In particular, we have been focusing on “collective phenomena”, namely, properties that the individual components comprising the interface (say a single molecule or an isolated inorganic substrate) do not exhibit, but the overall structure does. Perhaps the most striking example of such a collective effect is the possible emergence of magnetic phenomena at the interface between a nonmagnetic metal and a closedshell molecular layer. But there are many other examples, such as the emergence of qualitatively new electronic states at the interface, or highly delocalized excitons in organic materials. Furthermore, by no means is direct chemical interaction necessary for collective phenomena to occur. For example, longrange electrostatic effects can drastically affect the static polarization in, and electric fields outside of, a molecular monolayer. Consequently, properties of the molecular ensemble, as well as of its interface with the metal, can be very different from, and sometimes even opposite to, those of the isolated molecule.
Untangling the web of shortrange chemical bonds, longrange electrostatic and dispersive interactions, and intermolecular and moleculesubstrate interplay presents significant challenges. This is all the more so given that one has not only to understand general mechanisms that may allow for collective behavior, but also to rationalize how these depend on systemspecific properties such as bonding, order, and orientation, and understand when and how they may arise. Therefore, we mostly rely on first principles electronic structure theory, where electronic and optical properties are ideally deduced from nothing but the atomic species present and the laws of quantum physics.
Recent highlights of this line of research include:

Elucidating the nature of lowenergy electronic and optical excitations in prototypical organic electronic materials, highlighting strongly delocalized excitons.

T. Rangel, K. Berland, S. Sharifzadeh, F. Altvater, K. Lee, P. Hyldgaard, L. Kronik, and J. B. Neaton, “Structural and excitedstate properties of oligoacene crystals from first principles”, Phys. Rev. B 93, 115206 (2016).

S. Sharifzadeh, C. Y. Wong, H. Wu, B. L. Cotts, L. Kronik, N. S. Ginsberg, and J. B. Neaton, “Relating the physical structure and optoelectronic function of crystalline TIPSpentacene”, Adv. Funct. Mater. (Special Issue on Computational Modeling of Organic Semiconductors) 25, 2038 (2015).

S. Sharifzadeh, P. Darancet, L. Kronik, and J. B. Neaton, “LowEnergy ChargeTransfer Excitons in Organic Solids from FirstPrinciples: The Case of Pentacene”, J. Phys. Chem. Lett., 4, 2197 (2013).

S. Sharifzadeh, A. Biller, L. Kronik and J. B. Neaton, “Quasiparticle and optical spectroscopy of the organic semiconductors pentacene and PTCDA from first principles”, Phys. Rev. B 85, 125307 (2012).

T. Rangel, K. Berland, S. Sharifzadeh, F. Altvater, K. Lee, P. Hyldgaard, L. Kronik, and J. B. Neaton, “Structural and excitedstate properties of oligoacene crystals from first principles”, Phys. Rev. B 93, 115206 (2016).

Identification of the role of an interfaceinduced density of states at an organic/inorganic interface in interface energetics and charge transport across the interface.

(invited paper) T. Toledano, R. Garrick, O. Sinai, T. Bendikov, A. HajYahia, K. Lerman, H. Alon, C. Sukenik, A. Vilan, L. Kronik, D. Cahen, “Effect of binding group on hybridization across the silicon/aromaticmonolayer interface”, J. Electr. Spectr. Relat. Phenom. (Special Issue on Organic Electronics) 204, 149 (2015).

L. Kronik and Y. Morikawa, "Understanding the metalmolecule interface from first principles", in N. Koch, N. Ueno, and A. T. S. Wee, Ed., The MoleculeMetal Interface (WileyVCH, Weinheim, 2013),pp . 5189.

Y. Li, S. Calder, O. Yaffe, D. Cahen, H. Haick, L. Kronik, H. Zuilhof, “Hybrids of Organic Molecules and Flat, OxideFree Silicon: HighDensity Monolayers, Electronic properties, and Functionalization", Langmuir (Feature Article) 28, 9920 (2012).

O. Yaffe, T. Ely, R. HarLavan, D. Egger, S. Johnston, H. Cohen, L. Kronik, A. Vilan and D. Cahen, “Effect of MoleculeSurface Reaction Mechanism on the Electronic Characteristics and Photovoltaic Performance of MolecularlyModified Si”, J. Phys. Chem. C 117, 22351 (2013).

T. Aqua, H. Cohen, O. Sinai, V. Frydman, T. Bendikov, D. Krepel, O. Hod, L. Kronik and R. Naaman, “Role of backbone charge rearrangement in the bonddipole and work function of molecular monolayers”, J. Phys. Chem. C 115, 24888 (2011).

(invited paper) T. Toledano, R. Garrick, O. Sinai, T. Bendikov, A. HajYahia, K. Lerman, H. Alon, C. Sukenik, A. Vilan, L. Kronik, D. Cahen, “Effect of binding group on hybridization across the silicon/aromaticmonolayer interface”, J. Electr. Spectr. Relat. Phenom. (Special Issue on Organic Electronics) 204, 149 (2015).

Understanding how collective effects at the metal/molecule interface affect molecular electronics.
 D. Rakhmilevitch, S. Sarkar, O. Bitton, L. Kronik, and O. Tal, “Enhanced magnetoresistance in molecular junctions by geometrical optimization of spinselective orbital hybridization”, Nano Lett., 16, 1741 (2016).

Y. Li, P. Zolotavin, P. Doak, L. Kronik, J. B. Neaton, and D. Natelson, “Interplay of biasdriven charging and the vibrational Stark effect in molecular junctions”, Nano Lett. 16, 1104 (2016).

R. Vardimon, T. Yelin, M. Klionsky, S. Sarkar, A. Biller, L. Kronik, O. Tal, “Probing the Orbital Origin of Conductance Oscillations in Atomic Chains”, Nano Lett. 14, 2988 (2014).

Y. Li, P. Doak, L. Kronik, J, B. Neaton, D. Natelson, “Voltage tuning of vibrational mode energies in singlemolecule junctions”, PNAS 111, 1282 (2014).

T. Yelin, R. Vardimon, N. Kuritz, R. Korytár, A. Bagrets, F. Evers, L. Kronik and O. Tal, “Atomically wired molecular junctions: Connecting a single organic molecule by chains of metal atoms”, Nano. Lett. 13, 1956 (2013).

Understanding electrostatic, especially dipolar, properties of organic monolayers.

M. EckshtainLevi, E. Capua, S. RefaelyAbramson, S. Sarkar, Y. Gavrilov, S. Mathew, Y. Paltiel, Y. Levy, L. Kronik, and R. Naaman, “Cold Denaturation induces inversion of dipole and spin transfer in chiral peptide monolayers”, Nature Comm. 7, 10744 (2016).

F. Rissner, A. Natan, D. A. Egger, O. T. Hofmann. L. Kronik and E. Zojer, “Dimensionality effects in the electronic structure of organic semiconductors consisting of polar repeat units”, Org. Electr. 13, 3165 (2012).

F. Rissner, D. A. Egger, A. Natan, T. Körzdörfer, S. Kümmel, L. Kronik, and E. Zojer, “Collectively Induced QuantumConfined Stark Effect in Monolayers of Molecules Consisting of Polar Repeating Units”, J. Am. Chem. Soc. 133, 18634 (2011).

A. Natan, L. Kronik, H. Haick, and R. Tung, "Electrostatic Properties of Ideal and Nonideal Polar Organic Monolayers: Implications for Electronic Devices", Adv. Mater. 19, 4103 (2007).

M. EckshtainLevi, E. Capua, S. RefaelyAbramson, S. Sarkar, Y. Gavrilov, S. Mathew, Y. Paltiel, Y. Levy, L. Kronik, and R. Naaman, “Cold Denaturation induces inversion of dipole and spin transfer in chiral peptide monolayers”, Nature Comm. 7, 10744 (2016).

Understanding molecular level reorganization and renormalization at metal interfaces.

D. A. Egger, Z.F. Liu, J. B. Neaton, and L. Kronik, “Reliable Energy Level Alignment at Physisorbed MoleculeMetal Interfaces from Density Functional Theory”, Nano Lett. 15, 2448 (2015).

L. Kronik and Y. Morikawa, "Understanding the metalmolecule interface from first principles", in N. Koch, N. Ueno, and A. T. S. Wee, Ed., The MoleculeMetal Interface (WileyVCH, Weinheim, 2013),pp . 5189.

E. Salomon, P. Amsalem, N. Marom, M. Vondracek, L. Kronik, N. Koch, and T. Angot, “Electronic structure of CoPc adsorbed on Ag(100): evidence for moleculesubstrate interaction mediated by Co 3d orbitals", Phys. Rev. B 87, 075407 (2013).

A. Biller, I. Tamblyn, J. B. Neaton and L. Kronik, “Electronic level alignment at a metalmolecule interface from a shortrange hybrid functional”, J. Chem. Phys. 135, 164706 (2011).

D. A. Egger, Z.F. Liu, J. B. Neaton, and L. Kronik, “Reliable Energy Level Alignment at Physisorbed MoleculeMetal Interfaces from Density Functional Theory”, Nano Lett. 15, 2448 (2015).

In addition, we have developed advanced methods for the accurate prediction of electronic and optical properties of organic molecules and solids. [Click here for more information].
Organic−inorganic hybrids
Hybrid organic−inorganic perovskites (HOIPs) are crystals with the structural formula ABX3 where A, B, and X are organic and inorganic ions, respectively. While known for several decades, HOIPs have only in recent years emerged as extremely promising semiconducting materials for solar energy applications. In particular, power conversion efficiencies of HOIPbased solar cells have improved at a record speed and, after only little more than 6 years of photovoltaics research, surpassed the 20% threshold, which is an outstanding result for a solutionprocessable material. It is thus of fundamental importance to reveal physical and chemical phenomena that contribute to, or limit, these impressive photovoltaic efficiencies. To understand chargetransport and lightabsorption properties of semiconducting materials, one often invokes a lattice of ions displaced from their static positions only by harmonic vibrations. However, a preponderance of recent studies suggests that this picture is not sufficient for HOIPs, where a variety of structurally dynamic effects, beyond small harmonic vibrations, arises already at room temperature. Our research focuses on the theoretical understanding and prediction of such effects.
Recent highlights include:

General overview:
 T. M. Brenner, D. A. Egger, L. Kronik, G. Hodes, D. Cahen “Hybrid organic–inorganic perovskites: lowcost semiconductors with intriguing charge transport properties”, Nature Reviews Materials 1, 15007 (2016).

Overview of dynamic effects:

D. A. Egger, A. M. Rappe, and L. Kronik, “Hybrid OrganicInorganic Perovskites on the Move”, Acct. Chem. Research (Special Issue on LeadHalide Perovskites for Solar Energy Conversion), Acc. Chem. Res. 49, 573 (2016).

D. A. Egger, A. M. Rappe, and L. Kronik, “Hybrid OrganicInorganic Perovskites on the Move”, Acct. Chem. Research (Special Issue on LeadHalide Perovskites for Solar Energy Conversion), Acc. Chem. Res. 49, 573 (2016).

Discussion of opportunities and challenges:

J. Berry, T. Buonassisi, D. A. Egger, G. Hodes, L. Kronik, Y.L. Loo, I. Lubomirsky, S. R. Marder, Y. Mastai, J. S. Miller, D. B. Mitzi, Y. Paz, A. M. Rappe, I. Riess, B. Rybtchinski, O. Stafsudd, V. Stevanovic, M. F. Toney, D. Zitoun, A. Kahn, D. Ginley, D. Cahen, “Hybrid OrganicInorganic Perovskites (HOIPs): Opportunities and Challenges”, Adv. Mater. (Essay) 27, 5102 (2015).

J. Berry, T. Buonassisi, D. A. Egger, G. Hodes, L. Kronik, Y.L. Loo, I. Lubomirsky, S. R. Marder, Y. Mastai, J. S. Miller, D. B. Mitzi, Y. Paz, A. M. Rappe, I. Riess, B. Rybtchinski, O. Stafsudd, V. Stevanovic, M. F. Toney, D. Zitoun, A. Kahn, D. Ginley, D. Cahen, “Hybrid OrganicInorganic Perovskites (HOIPs): Opportunities and Challenges”, Adv. Mater. (Essay) 27, 5102 (2015).

Research articles:

S. Dastidar, D. A. Egger, L. Z. Tan, S. B. Cromer, A. D. Dillon, S. Liu, L. Kronik, A. M. Rappe, and A. T. Fafarman, “High Chloride Doping Levels Stabilize the Perovskite Phase of Cesium Lead Iodide”,Nano Lett. in press (2016).

T. M. Brenner, D. A. Egger, A. M. Rappe, L. Kronik, G. Hodes, and D. Cahen, “Are Mobilities in Hybrid OrganicInorganic Halide Perovskites Actually ‘High’?”, J. Phys. Chem. Lett. 6, 4754 (2015).

D. A. Egger, L. Kronik, and A. M. Rappe, “Theory of Hydrogen Migration in OrganicInorganic Halide Perovskites”, Angew. Chem. Int’l Ed. 54, 12437 (2015).
 D. A. Egger and L. Kronik, “Role of Dispersive Interactions in Determining Structural Properties of OrganicInorganic Halide Perovskites: Insights from First Principles Calculations”, J. Phys. Chem. Lett. 5, 2728 (2014).

S. Dastidar, D. A. Egger, L. Z. Tan, S. B. Cromer, A. D. Dillon, S. Liu, L. Kronik, A. M. Rappe, and A. T. Fafarman, “High Chloride Doping Levels Stabilize the Perovskite Phase of Cesium Lead Iodide”,Nano Lett. in press (2016).
Supramolecular materials
Supramolecular materials are defined as systems composed of molecules bound together by relatively weak intermolecular interactions, typically consisting of van der Waals (vdW) forces and/or hydrogen bonds. Supramolecular materials in general, and molecular solids in particular, play an important role in many areas of science and technology, ranging from mechanics and electronics to biology and medicine.
Molecular crystals often exhibit collective properties (i.e., properties arising from the weak intermolecular interactions) that are not found in the individual building blocks. See, e.g., the “organic molecular electronics” section for examples. Such properties are typically hard to predict from textbook molecular models. Therefore, firstprinciples calculations can be of great help in elucidating such phenomena.
Unfortunately, all standard approximations within density functional theory (DFT) fail to include longrange correlation expressions needed to account for weak interaction scenarios. Therefore, until recently DFT has scarcely left a footprint in the field of supramolecular materials.
In recent years, we have been using pairwise dispersive corrections, primarily those of TkatchenkoScheffler (for more information, see formalism section), to study such systems. We have shown that such corrections can be employed with any underlying exchangecorrelation functional, which allows us to achieve an unprecedented balance of accuracy of different binding scenarios, ranging from the very strong to the very weak. This allows us to use DFT to understand novel effects in molecular materials, that include unusual structural properties, unique mechanical properties, and a quantitative account of electronic and optical properties.
Recent highlights of this line of research include:

An overview of recent achievements and remaining challenges.

L. Kronik and A. Tkatchenko, “Understanding molecular crystals with dispersioninclusive densityfunctional theory: pairwise corrections and beyond”, Acct. Chem. Research (Special Issue on DFT Elucidation of Materials Properties), Acc. Chem. Res, in press.

L. Kronik and A. Tkatchenko, “Understanding molecular crystals with dispersioninclusive densityfunctional theory: pairwise corrections and beyond”, Acct. Chem. Research (Special Issue on DFT Elucidation of Materials Properties), Acc. Chem. Res, in press.

Theoretical prediction of a new polymorph of the guanine crystal, which is used for structural color determination in many organisms, followed by experimental confirmation of its dominance in biogenic guanine.

A. Hirsch, D. Gur, I. Polishchuk, D. Levy, B. Pokroy, A. J. CruzCabeza, L. Addadi, L. Kronik, and L. Leiserowitz, “‘Guanigma’: the revised structure of biogenic anhydrous guanine”, Chem. Mater. 27, 8289 (2015).

A. Hirsch, D. Gur, I. Polishchuk, D. Levy, B. Pokroy, A. J. CruzCabeza, L. Addadi, L. Kronik, and L. Leiserowitz, “‘Guanigma’: the revised structure of biogenic anhydrous guanine”, Chem. Mater. 27, 8289 (2015).

Theoretical prediction, followed by experimental confirmation, of unusually large and highly anisotropic Young’s moduli in amino acid molecular crystals, resulting from the hydrogenbonding network in these materials.

I. Azuri, E. Meirzadeh, D. Ehre, S. R. Cohen, A. M. Rappe, M. Lahav, I. Lubomirsky, and L. Kronik, “Unusually large Young's moduli of aminoacid molecular crystals”, Angew. Chem. Int’l Ed. 54, 13566 (2015).

I. Azuri, E. Meirzadeh, D. Ehre, S. R. Cohen, A. M. Rappe, M. Lahav, I. Lubomirsky, and L. Kronik, “Unusually large Young's moduli of aminoacid molecular crystals”, Angew. Chem. Int’l Ed. 54, 13566 (2015).

Understanding of the unique stiffness of diphenylalaninebased peptide nanostructures – a bioinspired supramolecular structure.

I. Azuri, L. AdlerAbramovich, E. Gazit, O. Hod, L. Kronik, “Why are diphenylalaninebased peptide nanostructures so rigid? Insights from first principles calculations”, J. Am. Chem. Soc., 136, 963 (2014).

I. Azuri, L. AdlerAbramovich, E. Gazit, O. Hod, L. Kronik, “Why are diphenylalaninebased peptide nanostructures so rigid? Insights from first principles calculations”, J. Am. Chem. Soc., 136, 963 (2014).

A complete understanding of the infrared spectrum of brushite – a crystalline hydrated acidic form of calcium phosphate that occurs in both physiological and pathological biomineralization processes.

A. Hirsch, I. Azuri, L. Addadi, S. Weiner, K. Yang, S. Curtarolo, L. Kronik, “Infrared Absorption Spectrum of Brushite from First Principles”, Chem. Mater., 26, 2934 (2014).

A. Hirsch, I. Azuri, L. Addadi, S. Weiner, K. Yang, S. Curtarolo, L. Kronik, “Infrared Absorption Spectrum of Brushite from First Principles”, Chem. Mater., 26, 2934 (2014).

Insights into the structure and formation of hemozoin – a molecular solid formed in the course of malaria.

N. Marom, A. Tkatchenko, S. Kapishnikov, L. Kronik and L. Leiserowitz, "Structure and Formation of Synthetic Hemozoin: Insights From FirstPrinciples Calculations", Cryst. Growth & Design 11, 3332 (2011).

N. Marom, A. Tkatchenko, S. Kapishnikov, L. Kronik and L. Leiserowitz, "Structure and Formation of Synthetic Hemozoin: Insights From FirstPrinciples Calculations", Cryst. Growth & Design 11, 3332 (2011).
 For an overview of the methodological developments that allow for these studies, Click here.
Biogenic Materials
Living organisms produce a wide range of materials, often revealing unique shapes, morphologies, structures, and functionality. Examples range from inorganic materials, such as calcium carbonates and phosphates used in, e.g., shells, bones, and teeth, to organic materials such as chitin, a polysaccharide used in the exoskeletons of arthropods, or a molecular solid of guanine, formed in the scales of some fish. In our work, we attempt to understand the unique order and structure in such materials, typically by comparing firstprinciples calculations to various spectroscopies.
Recent highlights include:

Understanding the relation between local order and infrared spectra of calcite.
 R. Gueta, A. Natan, L. Addadi, S. Weiner, K. Refson, and L. Kronik, "Local atomic order and infrared spectra of biogenic calcite", Angew. Chemie Int'l Ed. 46, 291 (2007).

K. M. Poduska, L. Regev, E. Boaretto, L. Addadi, S. Weiner, L. Kronik, and S. C. Curtarolo, "Decoupling local disorder and optical effects in infrared spectra: differentiating between calcites with different origins", Adv. Mater. 23, 550 (2011).

Assignment of the infrared spectrum of brushite, a crystalline hydrated acidic form of calcium phosphate that occurs in both physiological and pathological biomineralization processes.

A. Hirsch, I. Azuri, L. Addadi, S. Weiner, K. Yang, S. Curtarolo, L. Kronik, “Infrared Absorption Spectrum of Brushite from First Principles”, Chem. Mater. 26, 2934 (2014).

A. Hirsch, I. Azuri, L. Addadi, S. Weiner, K. Yang, S. Curtarolo, L. Kronik, “Infrared Absorption Spectrum of Brushite from First Principles”, Chem. Mater. 26, 2934 (2014).

Theoretical prediction of a new polymorph of the guanine crystal, which is used for structural color determination in many organisms, followed by experimental confirmation of its dominance in biogenic guanine.
 A. Hirsch, D. Gur, I. Polishchuk, D. Levy, B. Pokroy, A. J. CruzCabeza, L. Addadi, L. Kronik, and L. Leiserowitz, “‘Guanigma’: the revised structure of biogenic anhydrous guanine”, Chem. Mater. 27, 8289 (2015).
Orbitaldependent functionals
Optimallytuned rangeseparated hybrid functionals
Local hybrid functionals
Ensemblegeneralized functionals
Simulated photoelectron spectroscopy
Dispersion corrections
Simulated doping
Orbitaldependent functionals
First principles electronic structure calculations, based only on the periodic table and the laws of quantum mechanics, have made large strides in recent decades and have become the foundation for the understanding of a huge variety of physical and chemical systems.
Much of this progress has been due to density functional theory (DFT), which has emerged as the workhorse approach for realworld materials (as opposed to model systems). DFT is an approach to the manyelectron problem in which the electron density, rather than the manyelectron wave function, plays a central role. It has become the method of choice for electronic structure calculations across an unusually wide variety of fields, from organic chemistry to condensed matter physics. There are two main reasons for the spectacular success of DFT:

First and foremost, DFT offers the only currently known practical way for first principles calculations of systems with many thousands of electrons.

Second, it enhances our understanding by relying on relatively simple, accessible quantities that are easily visualized even for very large systems.
DFT has progressed from a formal approach to a practical one by virtue of the KohnSham equations. These constitute a mapping of the original Nelectron Schrödinger equation into an effective set of N oneelectron Schrödingerlike equations, where all nonclassical electron interactions (i.e., exchange and correlation) are subsumed into an additive oneelectron potential, known as the exchangecorrelation potential. The latter is the functional derivative of the exchangecorrelation energy, which is a functional of the density. This mapping is exact in principle, but always approximate in practice. Progress therefore hinges critically on our ability to obtain more accurate approximations for exchangecorrelation functionals that are applicable across a wide range of systems.
We believe that research into orbitaldependent density functionals is one of the most promising arenas in modern density functional theory. In such functionals, the exchangecorrelation energy is expressed explicitly in terms of KohnSham orbitals and is only an implicit functional of the density. This allows maximal freedom in functional construction and offers a real hope for alleviating some of the most serious difficulties associated with present day treatments of exchange and correlation within DFT. Furthermore, orbitaldependent functionals can be employed fully within the original KohnSham framework, in which case the exchangecorrelation potential is derived using the optimized effective potential equation. However, they can also be employed using the generalized KohnSham framework, in which case one obtains a nonlocal potential that corresponds to mapping the original manyelectron problem into one of partially interacting electrons. A leading example of those, although not always recognized as such, is the socalled hybrid functionals, where a fraction of exact exchange is “mixed in” with a fraction of explicitly densitydependent exchange. While the KohnSham mapping is unique, there are many generalized KohnSham maps. This additional flexibility allows one to choose the best mapping for a given task.
Our group is actively engaged in constructing, testing, benchmarking, and applying to complex systems several important classes of orbitaldependent functionals.

For a comprehensive review article on the topic, see:
 S. Kümmel and L. Kronik, "Orbitaldependent density functionals: theory and applications", Rev. Mod. Phys. 80, 3 (2008).
Optimallytuned rangeseparated hybrid functionals
In recent years we have been developing and employing functionals based on the concept of optimaltuning of a rangeseparated hybrid functional. In this approach, one separates the electronrepulsion into short and longrange components, treating the shortrange so as to achieve a good balance between exchange and correlation, using semilocal approximations (possibly with shortrange exactexchange), but emphasizing exactexchange in the longrange so as to obtain the correct asymptotic potential. Optimaltuning means that the rangeseparation parameter (roughly, the crossover point from short to long range) is an adjustable, systemdependent parameter (rather than a universal one). This parameter is obtained nonempirically based on the satisfaction of physical constraints, typically the ionization potential theorem and related properties. This allowed us to solve several related problems that plagued density functional theory, including the infamous gap problem (for finite systems) and the chargetransfer excitation problem.

For a perspectives article on this line of research, see
 L. Kronik, T. Stein, S. RefaelyAbramson, R. Baer, “Excitation Gaps of FiniteSized Systems from OptimallyTuned RangeSeparated Hybrid Functionals”, J. Chem. Theo. Comp. (Perspectives Article) 8, 1515 (2012).
Recent highlights of this line of research include:

Solving the chargetransfer excitation problem, in all its forms:

Full charge transfer:
 T. Stein, L. Kronik, and R. Baer, "Reliable Prediction of Charge Transfer Excitations in Molecular Complexes Using TimeDependent Density Functional Theory", J. Am. Chem. Soc. (Communications), 131, 2818 (2009).

Partial charge transfer:
 T. Stein, L. Kronik, and R. Baer, "Prediction of chargetransfer excitations in coumarinbased dyes using a rangeseparated functional tuned from first principles", J. Chem. Phys. 131, 244119 (2009).

Chargetransfer like scenarios:
 N. Kuritz, T. Stein , R. Baer and L. Kronik, "Chargetransferlike π → π* excitations in timedependent density functional theory: a conundrum and its solution", J. of Chem. Theory and Comp. 7, 2408 (2011).

Solving the gap problem for finite systems:
 S. RefaelyAbramson, R. Baer and L. Kronik, "Fundamental and excitation gaps in molecules of relevance for organic photovoltaics from an optimally tuned rangeseparated hybrid functional”, Phys. Rev. B 84, 075144 (2011). Selected as “Editor’s suggestion”.
 T. Stein, H. Eisenberg, L. Kronik, and R. Baer, "Fundamental gaps of finite systems from the eigenvalues of a generalized KohnSham method", Phys. Rev. Lett.,105, 266802 (2010).

Generalization to molecular solids
 S. RefaelyAbramson, M. Jain, S. Sharifzadeh, J. B. Neaton, L. Kronik, “Solidstate excitonic effects predicted from optimallytuned timedependent rangeseparated hybrid density functional theory”, Phys. Rev. B (Rapid Comm.) 92, 081204(R) (2015).
 D. Lüftner, S. RefaelyAbramson, M. Pachler, R. Resel, M. G. Ramsey, L. Kronik, and P. Puschnig, “Experimental and theoretical electronic structure of quinacridone”, Phys. Rev. B 90, 075204 (2014).
 S. RefaelyAbramson, S. Sharifzadeh, M. Jain, R. Baer, J. B. Neaton, L. Kronik, “Gap renormalization of molecular crystals from density functional theory”, Phys. Rev. B (Rapid Comm.), 88, 081204 (2013).

Generalization and application to outervalence electronic structure
 L. Kronik and S. Kümmel, "Gasphase valenceelectron photoemission spectroscopy using density functional theory”, in Topics of Current Chemistry: First Principles Approaches to Spectroscopic Properties of Complex Materials, C. di Valentin, S. Botti, M. Coccoccioni, Editors (Springer, Berlin, 2014), Volume 347, pp. 137192.
 D. Lüftner, S. RefaelyAbramson, M. Pachler, R. Resel, M. G. Ramsey, L. Kronik, and P. Puschnig, “Experimental and theoretical electronic structure of quinacridone”, Phys. Rev. B 90, 075204 (2014).
 D. A. Egger, S. Weismann, S. RefaelyAbramson, S. Sharifzadeh, M. Dauth, R. Baer, S. Kümmel, J. B. Neaton, E. Zojer, L. Kronik, “Outervalence electron spectra of prototypical aromatic heterocycles from an optimallytuned rangeseparated hybrid functional”, J. Chem. Theo. Comp., 10, 1934 (2014).
 S. RefaelyAbramson, S. Sharifzadeh, N, Govind, J. Autschbach, J. B. Neaton, R. Baer and L. Kronik, “Quasiparticle spectra from a nonempirical optimallytuned rangeseparated hybrid density functional”, Phys. Rev. Lett. 109, 226405 (2012).

Applications to amino acids and peptide structures
 (invited paper) S. Sarkar and L. Kronik, “Ionization and (de)protonation energies of gasphase amino acids from an optimally tuned rangeseparated hybrid functional”, Mol. Phys. (Special Issue in Honor of Professor Andreas Savin), in press (2016).
 M. EckshtainLevi, E. Capua, S. RefaelyAbramson, S. Sarkar, Y. Gavrilov, S. Mathew, Y. Paltiel, Y. Levy, L. Kronik, and R. Naaman, “Cold Denaturation induces inversion of dipole and spin transfer in chiral peptide monolayers”, Nature Comm. 7, 10744 (2016).
 L. Sepunaru, S. RefaelyAbramson, R. Lovrinčić, Y. Gavrilov, P. Agrawal, Y. Levy, L. Kronik, I. Pecht, M. Sheves, and D. Cahen, “Electronic Transport via Homopeptides: The Role of Side Chains and Secondary Structure”, J. Am. Chem. Soc. 137, 9617 (2015). Highlighted in Phys.org.

And finally some caveats
 A. Karolewski, L. Kronik, and S. Kümmel “Using optimallytuned range separated hybrid functionals in groundstate calculations: consequences and caveats”, J. Chem. Phys. 138, 204115 (2013).

Full charge transfer:
Local hybrid functionals
A different area of much interest to us is the development of novel local hybrid functionals, where the fraction of exact exchange is spatiallydependent, following variations in the density and orbitals. In particular, we are interested in using this to develop nonlocal correlation functionals compatible with exact exchange, that are free of oneelectron selfinteraction, respect constraints derived from uniform coordinate scaling, and exhibit the correct asymptotic behavior of the exchangecorrelation energy.

This approach is highlighted in:
 T. Schmidt, E. Kraisler, L. Kronik, S. Kümmel, “Oneelectron selfinteraction and the asymptotics of the KohnSham potential: an impaired relation”, Phys. Chem. Chem. Phys. 16, 14357 (2014).
 T. Schmidt, E. Kraisler, A. Makmal, L. Kronik, S. Kümmel, “A selfinteractionfree local hybrid functional: accurate binding energies visàvis accurate ionization potentials from KohnSham eigenvalues”, J. Chem. Phys. (Special issue on Density Functional Theory) 140, 18A510 (2014).
Ensemblegeneralized functionals
We have been heavily involved in the study of the derivative discontinuity – a quirky property of the exchangecorrelation potential, which makes it “jump” by a constant across an integer number of electrons. Traditionally, simple approximations to the exchangecorrelation functional were thought to be devoid of this derivative discontinuity, with severe implications to the computation of bandgaps from eigenvalues, where this must be taken into account. Recently, we have discovered that in fact with a simple ensemblegeneralization all functionals possess a derivative discontinuity and that this can be used to extract relatively accurate molecular gaps even from very simple functionals.
Recent highlights of this line of research include:

Demonstrating the existence and importance of the derivative discontinuity in the potential of a dissociating molecule:
 A. Makmal, S. Kümmel, and L. Kronik, "Dissociation of diatomic molecules and the exactexchange KohnSham potential: the case of LiF", Phys. Rev. A 83, 062512 (2011).

Revealing the derivative discontinuity from ensemble considerations, including implications for orbital energies and gaps:
 E. Kraisler and L. Kronik, “Fundamental gaps with approximate density functionals: the derivative discontinuity revealed from ensemble considerations”, J. Chem. Phys. 140, 18A540 (2014).
 T. Schmidt, E. Kraisler, L. Kronik, S. Kümmel, “Oneelectron selfinteraction and the asymptotics of the KohnSham potential: an impaired relation” Phys. Chem. Chem. Phys. 16, 14357 (2014).
 E. Kraisler and L. Kronik, “Piecewise linearity of approximate density functionals revisited: Implications for frontier orbital energies”, Phys. Rev. Lett. 110, 126403 (2013).
 E. Kraisler, T. Schmidt, S. Kümmel, and L. Kronik, “Effect of ensemble generalization on the highestoccupied KohnSham eigenvalue”, J. Chem. Phys. 143, 104105 (2015).
Simulated photoelectron spectroscopy
We have been exploring the pros and cons of using various functionals, including explicitly densitydependent ones, and different conventional and novel hybrid ones, as a tool for quantitative simulations of photoelectron spectroscopy.
Recent highlights of this line of research include:

For an overview article, see:
 L. Kronik and S. Kümmel, "Gasphase valenceelectron photoemission spectroscopy using density functional theory”, in Topics of Current Chemistry: First Principles Approaches to Spectroscopic Properties of Complex Materials, C. di Valentin, S. Botti, M. Coccoccioni, Editors (Springer, Berlin, 2014).

For investigations of phthaolcyanines, see:
 N. Marom, O. Hod, G. E. Scuseria, and L. Kronik, "Electronic Structure of Copper Phthalocyanine: a Comparative Density Functional Theory Study", J. Chem. Phys. 128, 164107 (2008).
 (invited paper) N. Marom and L. Kronik, "Density Functional Theory of Transition Metal Phthalocyanines. I: Electronic Structure of NiPc and CoPc SelfInteraction Effects", Appl. Phys. A 95, 159 (2009). (Special Issue on Organic Materials for Electronic Applications).
 (invited paper) N. Marom and L. Kronik, "Density Functional Theory of Transition Metal Phthalocyanines. II: Electronic Structure of MnPc and FePc  Symmetry and Symmetry Breaking", Appl. Phys. A 95, 165 (2009). (Special Issue on Organic Materials for Electronic Applications)
 D. A. Egger, S. Weismann, S. RefaelyAbramson, S. Sharifzadeh, M. Dauth, R. Baer, S. Kümmel, J. B. Neaton, E. Zojer, L. Kronik, “Outervalence electron spectra of prototypical aromatic heterocycles from an optimallytuned rangeseparated hybrid functional”, J. Chem. Theo. Comp. 10, 1934 (2014).

For investigation of aromatic molecules and their derivatives, see:
 D. Lüftner, S. RefaelyAbramson, M. Pachler, R. Resel, M. G. Ramsey, L. Kronik, and P. Puschnig, “Experimental and theoretical electronic structure of quinacridone”, Phys. Rev. B 90, 075204 (2014).
 S. RefaelyAbramson, S. Sharifzadeh, N, Govind, J. Autschbach, J. B. Neaton, R. Baer and L. Kronik, “Quasiparticle spectra from a nonempirical optimallytuned rangeseparated hybrid density functional”, Phys. Rev. Lett. 109, 226405 (2012).
 T. Körzdörfer, S. Kümmel, N. Marom, and L. Kronik, "When to trust photoelectron spectra from KohnSham eigenvalues: the case of organic semiconductors", Phys. Rev. B (Rapid Comm.) 79, 201205 (2009).
 N. Dori, M. Menon, L. Kilian, M. Sokolowski, L. Kronik, and E. Umbach, "Valence Electronic Structure of Gas Phase 3,4,9,10perylene tetracarboxylicaciddianhydride (PTCDA): Experiment and Theory", Phys. Rev. B 73, 195208 (2006).
Dispersion corrections
Last but not at all least, we are interested in further developments and applications of pairwise and beyondpairwise dispersion corrections both standard and optimallytuned rangeseparated hybrid functionals, as a means of incorporating longrange correlation that is essential to the capture of weak interactions.
Recent highlights of this line of research include:

For an overview article, see:
 L. Kronik and A. Tkatchenko, “Understanding molecular crystals with dispersioninclusive densityfunctional theory: pairwise corrections and beyond”, Acct. Chem. Research (Special Issue on DFT Elucidation of Materials Properties), Acc. Chem. Res, in press.

For a combination of this approach with optimallytuned rangeseparated hybrid functionals, see:
 P. Agrawal, A. Tkatchenko, and L. Kronik, “Pairwise and manybody dispersive interactions coupled to an optimallytuned rangeseparated hybrid functional", J. Chem. Theo. Comp., 9, 3473 (2013).
Simulated doping
The inclusion of the global effects of semiconductor doping poses a unique challenge for firstprinciples simulations, because the typically low concentration of dopants renders an explicit treatment intractable. Furthermore, the width of the spacecharge region (SCR) at charged surfaces often exceeds realistic supercell dimensions. Recently, we developed a multiscale technique that fully addresses these difficulties. It is based on the introduction of a charged sheet, mimicking the SCRrelated field, along with free charge which mimics the bulk charge reservoir, such that the system is neutral overall. These augment a slab comprising “pseudoatoms” possessing a fractional nuclear charge matching the bulk doping concentration. Selfconsistency is reached by imposing charge conservation and Fermi level equilibration between the bulk, treated semiclassically, and the electronic states of the slab, which are treated quantummechanically. The method, called CREST—the chargereservoir electrostatic sheet technique—can be used with standard electronic structure codes.

For recent highlights of this approach see:
 O. Sinai, O. T. Hofmann, P. Rinke, M. Scheffler, G. Heimel, and L. Kronik, “Multiscale approach to the electronic structure of doped semiconductor surfaces”, Phys. Rev. B 91, 075311 (2015). Selected as “Editor’s suggestion”.
 O. Sinai and L. Kronik, ”Simulated doping of Si from first principles using pseudoatoms", Phys. Rev. B 87, 235305 (2013).