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Burgess AC, Linscott E, O'Regan DD. The convexity condition of density-functional theory. J Chem Phys 2023; 159:211102. [PMID: 38038199 DOI: 10.1063/5.0174159] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/28/2023] [Accepted: 11/08/2023] [Indexed: 12/02/2023] Open
Abstract
It has long been postulated that within density-functional theory (DFT), the total energy of a finite electronic system is convex with respect to electron count so that 2Ev[N0] ≤ Ev[N0 - 1] + Ev[N0 + 1]. Using the infinite-separation-limit technique, this Communication proves the convexity condition for any formulation of DFT that is (1) exact for all v-representable densities, (2) size-consistent, and (3) translationally invariant. An analogous result is also proven for one-body reduced density matrix functional theory. While there are known DFT formulations in which the ground state is not always accessible, indicating that convexity does not hold in such cases, this proof, nonetheless, confirms a stringent constraint on the exact exchange-correlation functional. We also provide sufficient conditions for convexity in approximate DFT, which could aid in the development of density-functional approximations. This result lifts a standing assumption in the proof of the piecewise linearity condition with respect to electron count, which has proven central to understanding the Kohn-Sham bandgap and the exchange-correlation derivative discontinuity of DFT.
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Affiliation(s)
- Andrew C Burgess
- School of Physics, Trinity College Dublin, The University of Dublin, Dublin, Ireland
| | - Edward Linscott
- Theory and Simulation of Materials (THEOS), École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland
| | - David D O'Regan
- School of Physics, Trinity College Dublin, The University of Dublin, Dublin, Ireland
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2
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Gould T. Toward routine Kohn-Sham inversion using the "Lieb-response" approach. J Chem Phys 2023; 158:064102. [PMID: 36792495 DOI: 10.1063/5.0134330] [Citation(s) in RCA: 3] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/10/2023] Open
Abstract
Kohn-Sham (KS) inversion, in which the effective KS mean-field potential is found for a given density, provides insights into the nature of exact density functional theory (DFT) that can be exploited for the development of density functional approximations. Unfortunately, despite significant and sustained progress in both theory and software libraries, KS inversion remains rather difficult in practice, especially in finite basis sets. The present work presents a KS inversion method, dubbed the "Lieb-response" approach, that naturally works with existing Fock-matrix DFT infrastructure in finite basis sets, is numerically efficient, and directly provides meaningful matrix and energy quantities for pure-state and ensemble systems. Some additional work yields potential. It thus enables the routine inversion of even difficult KS systems, as illustrated in a variety of problems within this work, and provides outputs that can be used for embedding schemes or machine learning of density functional approximations. The effect of finite basis sets on KS inversion is also analyzed and investigated.
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Affiliation(s)
- Tim Gould
- Qld Micro- and Nanotechnology Centre, Griffith University, Nathan, Qld 4111, Australia
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3
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Gould T, Hashimi Z, Kronik L, Dale SG. Single Excitation Energies Obtained from the Ensemble "HOMO-LUMO Gap": Exact Results and Approximations. J Phys Chem Lett 2022; 13:2452-2458. [PMID: 35266399 DOI: 10.1021/acs.jpclett.2c00042] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/14/2023]
Abstract
In calculations based on density functional theory, the "HOMO-LUMO gap" (difference between the highest occupied and lowest unoccupied molecular orbital energies) is often used as a low-cost, ad hoc approximation for the lowest excitation energy. Here we show that a simple correction based on rigorous ensemble density functional theory makes the HOMO-LUMO gap exact in principle and significantly more accurate in practice. The introduced perturbative ensemble density functional theory approach predicts different and useful values for singlet-singlet and singlet-triplet excitations, using semilocal and hybrid approximations. Excitation energies are similar in quality to time-dependent density functional theory, especially at high fractions of exact exchange. The approach therefore offers an easy-to-implement and low-cost route to robust prediction of molecular excitation energies.
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Affiliation(s)
- Tim Gould
- Queensland Micro- and Nanotechnology Centre, Griffith University, Nathan, QLD 4111, Australia
| | - Zahed Hashimi
- Queensland Micro- and Nanotechnology Centre, Griffith University, Nathan, QLD 4111, Australia
| | - Leeor Kronik
- Department of Molecular Chemistry and Materials Science, Weizmann Institute of Science, Rehovoth 76100, Israel
| | - Stephen G Dale
- Queensland Micro- and Nanotechnology Centre, Griffith University, Nathan, QLD 4111, Australia
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4
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Callow T, Pearce B, Gidopoulos N. Density functionals with spin-density accuracy for open shells. J Chem Phys 2022; 156:111101. [DOI: 10.1063/5.0071991] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
Electrons in zero external magnetic field can be studied with density functional theory (DFT) or with spin-DFT (SDFT). The latter is normally used for open shell systems because its approximations appear to model better the exchange and correlation (xc) functional, but also because so far the application of DFT implied a closed-shell-like approximation. Correcting this error for open shells allows the approximate DFT xc functionals to become as accurate as those in SDFT. In the limit of zero magnetic field, the Kohn-Sham equations of SDFT emerge as the generalised KS equations of DFT.
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Affiliation(s)
| | | | - Nikitas Gidopoulos
- Department of Physics, Durham University Department of Physics, United Kingdom
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5
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Kumar A, Harbola MK. Using random numbers to obtain Kohn-Sham potential for a given density. Chem Phys Lett 2021. [DOI: 10.1016/j.cplett.2021.138851] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/21/2022]
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6
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Gould T, Kronik L. Ensemble generalized Kohn-Sham theory: The good, the bad, and the ugly. J Chem Phys 2021; 154:094125. [PMID: 33685152 DOI: 10.1063/5.0040447] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
Two important extensions of Kohn-Sham (KS) theory are generalized: KS theory and ensemble KS theory. The former allows for non-multiplicative potential operators and greatly facilitates practical calculations with advanced, orbital-dependent functionals. The latter allows for quantum ensembles and enables the treatment of open systems and excited states. Here, we combine the two extensions, both formally and practically, first via an exact yet complicated formalism and then via a computationally tractable variant that involves a controlled approximation of ensemble "ghost interactions" by means of an iterative algorithm. The resulting formalism is illustrated using selected examples. This opens the door to the application of generalized KS theory in more challenging quantum scenarios and to the improvement of ensemble theories for the purpose of practical and accurate calculations.
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Affiliation(s)
- Tim Gould
- QLD Micro- and Nanotechnology Centre, Griffith University, Nathan, QLD 4111, Australia
| | - Leeor Kronik
- Department of Materials and Interfaces, Weizmann Institute of Science, Rehovoth 76100, Israel
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7
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Kraisler E, Hodgson MJP, Gross EKU. From Kohn-Sham to Many-Electron Energies via Step Structures in the Exchange-Correlation Potential. J Chem Theory Comput 2021; 17:1390-1407. [PMID: 33595312 PMCID: PMC8363072 DOI: 10.1021/acs.jctc.0c01093] [Citation(s) in RCA: 9] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
![]()
Accurately
describing excited states within Kohn–Sham (KS)
density functional theory (DFT), particularly those which induce ionization
and charge transfer, remains a great challenge. Common exchange-correlation
(xc) approximations are unreliable for excited states owing, in part,
to the absence of a derivative discontinuity in the xc energy (Δ),
which relates a many-electron energy difference to the corresponding
KS energy difference. We demonstrate, analytically and numerically,
how the relationship between KS and many-electron energies leads to
the step structures observed in the exact xc potential in four scenarios:
electron addition, molecular dissociation, excitation of a finite
system, and charge transfer. We further show that steps in the potential
can be obtained also with common xc approximations, as simple as the
LDA, when addressed from the ensemble perspective. The article therefore
highlights how capturing the relationship between KS and many-electron
energies with advanced xc approximations is crucial for accurately
calculating excitations, as well as the ground-state density and energy
of systems which consist of distinct subsystems.
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Affiliation(s)
- Eli Kraisler
- Fritz Haber Center for Molecular Dynamics and Institute of Chemistry, The Hebrew University of Jerusalem, 9091401 Jerusalem, Israel
| | - M J P Hodgson
- Department of Physics, Durham University, South Road, Durham DH1 3LE, United Kingdom.,Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2, D-06120 Halle, Germany
| | - E K U Gross
- Fritz Haber Center for Molecular Dynamics and Institute of Chemistry, The Hebrew University of Jerusalem, 9091401 Jerusalem, Israel
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8
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Gould T, Liberles BT, Perdew JP. What do we learn from the classical turning surface of the Kohn-Sham potential as electron number is varied continuously? J Chem Phys 2020; 152:054105. [PMID: 32035475 DOI: 10.1063/1.5130693] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022] Open
Abstract
The classical Kohn-Sham turning radius Rt of an atom can be defined as the radius where the Kohn-Sham potential is equal to the negative ionization potential of the atom, i.e., where vs(Rt) = ϵh. It was recently shown [E. Ospadov et al., Proc. Natl. Acad. Sci. U. S. A. 115, E11578-E11585 (2018)] to yield chemically relevant bonding distances, in line with known empirical values. In this work, we show that extension of the concept to non-integer electron number yields additional information about atomic systems and can be used to detect the difficulty of adding or subtracting electrons. Notably, it reflects the ease of bonding in open p-shells and its greater difficulty in open s-shells. The latter manifests in significant discontinuities in the turning radius as the electron number changes the principal quantum number of the outermost electronic shell (e.g., going from Na to Na2+). We then show that a non-integer picture is required to correctly interpret bonding and dissociation in H2 +. Results are consistent when properties are calculated exactly or via an appropriate approximation. They can be interpreted in the context of conceptual density functional theory.
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Affiliation(s)
- Tim Gould
- Qld Micro- and Nanotechnology Centre, Griffith University, Nathan, Qld 4111, Australia
| | | | - John P Perdew
- Department of Physics, Temple University, Philadelphia, Pennsylvania 19122, USA
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9
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Kraisler E. Asymptotic Behavior of the Exchange‐Correlation Energy Density and the Kohn‐Sham Potential in Density Functional Theory: Exact Results and Strategy for Approximations. Isr J Chem 2020. [DOI: 10.1002/ijch.201900103] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
Affiliation(s)
- Eli Kraisler
- Fritz Haber Center for Molecular Dynamics and Institute of Chemistry The Hebrew University of Jerusalem 9091401 Jerusalem Israel
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10
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Lonsdale DR, Goerigk L. The one-electron self-interaction error in 74 density functional approximations: a case study on hydrogenic mono- and dinuclear systems. Phys Chem Chem Phys 2020; 22:15805-15830. [PMID: 32458849 DOI: 10.1039/d0cp01275k] [Citation(s) in RCA: 17] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
The self-interaction error (SIE), i.e. unphysical interactions of electrons with themselves, has plagued developers and users of Density Functional Approximations (DFAs) since the inception of Density Functional Theory (DFT). Formally, it can be separated into the one-electron and many-electron SIE; herein we present one of the most comprehensive studies of the first. While we focus mostly on the total SIE, we also make use of two different decompositions. The first is a separation into functional and density-driven errors as championed by Sim, Burke and co-workers [J. Phys. Chem. Lett., 2018, 9, 6385-6392]; the second separates the error into exchange, correlation, and one-electron components, with the latter being a density error that has not been discussed in this form before. After investigating the familiar hydrogen atom and dihydrogen cation, we establish a relationship between the SIE and the nuclear charge with the help of a series of heavier hydrogenic analogues. For the mononuclear systems and the diatomics at the dissociation limit, this relationship is linear in nature with prominent exceptions, mostly belonging to the Minnesota and range-separated (double-)hybrid DFAs. For the first time, we also show how the magnitude of the SIE depends on the underlying atomic-orbital basis set and how DFAs that rely on a popular van-der-Waals DFT type London-dispersion term exhibit "self-dispersion". We find that range separation is not a panacea for solving the one-electron SIE. DFAs that have been developed to be one-electron SIE free for one system, such as the hydrogen atom, show larger errors for heavier hydrogenic systems. Often, one-electron SIE-free DFAs rely on fortuitous error cancellation between their exchange and correlation components. An analysis of the most robust methods for general applications to date reveals that they suffer moderately from the one-electron SIE, while DFAs that are nearly SIE-free do not perform well in applications. Implicit in the continued existence of the one-electron SIE is that well-performing DFAs continue to suffer insufficiencies at their fundamental levels that are being compensated for by the SIE. Our analysis includes more than 250 000 datapoints, resulting in multiple insights that may drive future developments of new DFAs or SIE corrections.
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Affiliation(s)
- Dale R Lonsdale
- School of Chemistry, The University of Melbourne, Parkville, Australia.
| | - Lars Goerigk
- School of Chemistry, The University of Melbourne, Parkville, Australia.
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11
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Kronik L, Kümmel S. Piecewise linearity, freedom from self-interaction, and a Coulomb asymptotic potential: three related yet inequivalent properties of the exact density functional. Phys Chem Chem Phys 2020; 22:16467-16481. [DOI: 10.1039/d0cp02564j] [Citation(s) in RCA: 22] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022]
Abstract
Three properties of the exact energy functional of DFT are important in general and for spectroscopy in particular, but are not necessarily obeyed by approximate functionals. We explain what they are, why they are important, and how they are related yet inequivalent.
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Affiliation(s)
- Leeor Kronik
- Department of Materials and Interfaces
- Weizmann Institute of Science
- Rehovoth 76100
- Israel
| | - Stephan Kümmel
- Theoretical Physics IV
- University of Bayreuth
- 95440 Bayreuth
- Germany
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12
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Gould T, Pittalis S. Density-Driven Correlations in Ensemble Density Functional Theory: Insights from Simple Excitations in Atoms. Aust J Chem 2020. [DOI: 10.1071/ch19504] [Citation(s) in RCA: 9] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
Abstract
Ensemble density functional theory extends the usual Kohn-Sham machinery to quantum state ensembles involving ground- and excited states. Recent work by the authors [Phys. Rev. Lett. 119, 243001 (2017); 123, 016401 (2019)] has shown that both the Hartree-exchange and correlation energies can attain unusual features in ensembles. Density-driven (DD) correlations – which account for the fact that pure-state densities in Kohn-Sham ensembles do not necessarily reproduce those of interacting pure states – are one such feature. Here we study atoms (specifically S–P and S–S transitions) and show that the magnitude and behaviour of DD correlations can vary greatly with the variation of the orbital angular momentum of the involved states. Such estimations are obtained through an approximation for DD correlations built from relevant exact conditions, Kohn-Sham inversion, and plausible assumptions for weakly correlated systems.
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