The Experts below are selected from a list of 327 Experts worldwide ranked by ideXlab platform
David J. Tozer - One of the best experts on this subject based on the ideXlab platform.
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approximating the shifted hartree exchange correlation Potential in direct energy kohn sham theory
Journal of Chemical Theory and Computation, 2018Co-Authors: Daniel Joshua Sharpe, Mel Levy, David J. TozerAbstract:Levy and Zahariev [Phys. Rev. Lett. 113 113002 (2014)] have proposed a new approach for performing density functional theory calculations, termed direct energy Kohn–Sham (DEKS) theory. In this approach, the electronic energy equals the sum of orbital energies, obtained from Kohn–Sham-like orbital equations involving a shifted Hartree-Exchange-Correlation Potential, which must be approximated. In the present study, density scaling homogeneity considerations are used to facilitate DEKS calculations on a series of atoms and molecules, leading to three nonlocal approximations to the shifted Potential. The first two rely on preliminary Kohn–Sham calculations using a standard generalized gradient approximation (GGA) Exchange-Correlation functional and the results illustrate the benefit of describing the dominant Hartree component of the shift exactly. A uniform electron gas analysis is used to eliminate the need for these preliminary Kohn–Sham calculations, leading to a Potential with an unconventional form tha...
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Orbital energies and negative electron affinities from density functional theory: Insight from the integer discontinuity.
The Journal of Chemical Physics, 2008Co-Authors: Andrew M. Teale, Frank De Proft, David J. TozerAbstract:Orbital energies in Kohn–Sham density functional theory (DFT) are investigated, paying attention to the role of the integer discontinuity in the exact Exchange-Correlation Potential. A series of closed-shell molecules are considered, comprising some that vertically bind an excess electron and others that do not. High-level ab initio electron densities are used to calculate accurate orbital energy differences, Δϵ, between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO), using the same Potential for both. They are combined with accurate vertical ionization Potentials, I0, and electron affinities, A0, to determine accurate “average” orbital energies. These are the orbital energies associated with an Exchange-Correlation Potential that averages over a constant jump in the accurate Potential, of magnitude ΔXC=(I0−A0)−Δϵ, as given by the discontinuity analysis. Local functional HOMO energies are shown to be almost an order of magnitude closer to these average val...
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The importance of the asymptotic Exchange-Correlation Potential in density functional theory
Molecular Physics, 2003Co-Authors: David J. Tozer, Nicholas C. HandyAbstract:The importance to density functional theory (DFT) of the integer discontinuity in the Exchange-Correlation Potential is reiterated and further examined. It follows that if generalized gradient approximation (GGA) functional are used, then the asymptotic value of the Exchange-Correlation Potential must be a positive system-dependent constant. Our previously introduced asymptotic correction (AC), which forces the correct behaviour of the Potential in the asymptotic region, is further discussed. Three examples are given that demonstrate the importance of the analysis; (i) HOMO eigenvalues lie well above the negative of the ionization energy when GGA functionals are used—this is not detrimental to GGA DFT; (ii) an independent calculation of the GGA asymptotic Potentials for some molecules is presented, based on the determination of parametrized GGA functionals; (iii) Rydberg states of He+ and H+ 2 are studied, demonstrating that essentially exact Rydberg Potential energy curves can be determined if the AC is ...
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The Exchange-Correlation Potential in Kohn–Sham nuclear magnetic resonance shielding calculations
The Journal of Chemical Physics, 2003Co-Authors: Thomas W Keal, David J. TozerAbstract:A simple gradient correction to the local density approximation functional is proposed, which improves the structure of the Exchange-Correlation Potential. The optimized generalized gradient approximation (GGA) functional provides uncoupled isotropic and anisotropic nuclear magnetic resonance shielding constants that are 2–3 times more accurate than those of commonly used GGAs, for a series of challenging molecules involving first- and second-row atoms; the results are competitive with those of ab initio wave function methods. A correlation is observed between the lowest occupied-virtual eigenvalue difference and the shielding accuracy. Magnetizabilities are also improved. The performance of the functional for structural and energetic predictions is investigated. These properties can be improved by relaxing the uniform electron gas condition, with no degradation in shielding quality. Atomization energies, ionization Potentials, and molecular bond lengths are then comparable to those of other GGA functiona...
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the exchange correlation Potential in kohn sham nuclear magnetic resonance shielding calculations
Journal of Chemical Physics, 2003Co-Authors: Thomas W Keal, David J. TozerAbstract:A simple gradient correction to the local density approximation functional is proposed, which improves the structure of the Exchange-Correlation Potential. The optimized generalized gradient approximation (GGA) functional provides uncoupled isotropic and anisotropic nuclear magnetic resonance shielding constants that are 2–3 times more accurate than those of commonly used GGAs, for a series of challenging molecules involving first- and second-row atoms; the results are competitive with those of ab initio wave function methods. A correlation is observed between the lowest occupied-virtual eigenvalue difference and the shielding accuracy. Magnetizabilities are also improved. The performance of the functional for structural and energetic predictions is investigated. These properties can be improved by relaxing the uniform electron gas condition, with no degradation in shielding quality. Atomization energies, ionization Potentials, and molecular bond lengths are then comparable to those of other GGA functiona...
N. H. March - One of the best experts on this subject based on the ideXlab platform.
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the exchange correlation Potential of dft obtained from a semiempirically fine tuned hartree fock density for inhomogeneous electron liquids
Physics and Chemistry of Liquids, 2012Co-Authors: Claudio Amovilli, N. H. MarchAbstract:The present authors have given an exact theory of the Exchange-Correlation Potential V xc (r) in terms of (i) the exact ground-state electron density n(r) and (ii) the idempotent Dirac density matrix γ(r, r′) generated by the DFT one-body Potential V(r), having n(r) as its diagonal element. Here, we display two approximate consequences: (a) a form of V xc (r) generated by the semiempirically fine-tuned HF density of Cordero et al. (N.A. Cordero, N.H. March, and J.A. Alonso, Phys. Rev. A 75, 052502 (2007)) and (b) the exchange-only Potential V x (r) determined solely by the HF ground state density for the Be atom.
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X-ray scattering factor of the inhomogeneous electron liquid in the ground state of the Ne atom: contribution of the Exchange-Correlation Potential
Physics and Chemistry of Liquids, 2008Co-Authors: C. Van Alsenoy, N. H. MarchAbstract:We have recently set out an approach to the X-ray scattering from 10-electron central field systems such as the almost spherical molecular species CH4 and NH , in terms of DFT. Here, we take the united atom Ne of CH4 as our focal point, and exploit the fact that the Exchange-Correlation Potential V xc (r) is known from the work of Zhao et al. (Q. Zhao, R.C. Morrison and R.G. Parr, Phys. Rev. A50, 2138 (1994)). Thus, we can calculate the explicit and sizeable contribution that the Exchange-Correlation force −∂V xc /∂r derived from this Exchange-Correlation Potential makes to the measured X-ray scattering factor of Ne.
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Exchange-Correlation Potential in terms of the idempotent Dirac density matrix of DFT
Chemical Physics Letters, 2006Co-Authors: Claudio Amovilli, N. H. MarchAbstract:Abstract A formal proof has recently been proposed [I.A. Howard, N.H. March, Chem. Phys. Lett. 402 (2005) 1.] to show that the Exchange-Correlation Potential V xc ( r ) of density functional theory (DFT) is, in principle, determined by the idempotent Dirac density matrix γ ( r , r ′) generated by the one-body Potential V ( r ) = V ext ( r ) + V Hartree ( r ) + V xc ( r ). Here we turn this ‘existence’ theorem into a concrete result by expressing the Laplacian ∇ 2 V xc ( r ) solely in terms of γ ( r , r ′), which, although idempotent and therefore not many-electron in character, has the exact ground state density ρ ( r ) on its diagonal γ ( r , r ). As a specific illustration, we calculate ρ ( r ) by diffusion QMC and hence γ ( r , r ′) for the Be atom ground-state. V xc ( r ) is thereby obtained with satisfactory accuracy.
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Force -∂Vxc/∂r associated with the Exchange-Correlation Potential Vxc(r) in the neutral Ne atom
Journal of Physics B: Atomic Molecular and Optical Physics, 2005Co-Authors: I. A. Howard, K D Sen, Paul Geerlings, N. H. MarchAbstract:Following an electrostatic interpretation of the force Fxc = −∂Vxc/∂r associated with the Exchange-Correlation Potential Vxc(r), we present both analytical and numerical results for Fxc(r) in the Ne atom. The basic input is an existing quantum Monte Carlo (QMC) calculation of the ground-state electron density ρ(r) in this atom. The analytic form of ∂Vxc/∂r is in terms of the number of electrons Q(r) enclosed in a sphere of radius r centred on the nucleus, plus two phases needed to characterize the radial wavefunctions of density functional theory. Eigenvalue equations are presented for these phases, and used numerically. A brief discussion is added on the result of replacing the QMC ground-state density by its Hartree–Fock counterpart.
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electrostatic interpretation of the force vxc r connected with the exchange correlation Potential direct relation to single particle kinetic energy density in be atom
Physics Letters A, 2004Co-Authors: N. H. March, Paul Geerlings, K D SenAbstract:Abstract In a recent study by one of us [Phys. Rev. A 65 (2002) 034501], an electrostatic interpretation has been proposed for the force − ∂V xc / ∂r associated with the Exchange-Correlation Potential energy V xc ( r ) in a spherical atom such as Be or Ne. Here, this proposal is employed to relate − ∂V xc / ∂r directly to (a) the exact ground-state density ρ ( r ) (e.g., from diffraction experiments or quantum Monte Carlo simulation) and its low-order derivatives, and (b) the single-particle kinetic energy density t s ( r ), calculated again from the exact ρ ( r ), for the example of the Be-atom. It would, of course, be important if this type of relation could be extended beyond a (1s) 2 (2s) 2 two-level single-particle configuration.
Tom Ziegler - One of the best experts on this subject based on the ideXlab platform.
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the performance of time dependent density functional theory based on a noncollinear exchange correlation Potential in the calculations of excitation energies
Journal of Chemical Physics, 2005Co-Authors: Fan Wang, Tom ZieglerAbstract:In the present work we have studied the accuracy of excitation energies calculated from spin-flip transitions with a formulation of time-dependent density functional theory based on a noncollinear Exchange-Correlation Potential proposed in a previous study. We compared the doublet-doublet excitation energies from spin-flip transitions and ordinary transitions, calculated the multiplets splitting of some atoms, the singlet-triplet gaps of some diradicals, the energies of excited quartet states with a doublet ground state. In addition, we attempted to calculate transition energies with excited states as reference. We compared the triplet excitation energies and singlet-triplet separations of the excited state from spin-flip and ordinary transitions. As an application, we show that using excited quartet state as reference can help us fully resolve excited states spin multiplets. In total the obtained excitation energies calculated from spin-flip transitions agree quite well with other theoretical results or experimental data.
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Time-dependent density functional theory based on a noncollinear formulation of the Exchange-Correlation Potential.
The Journal of chemical physics, 2004Co-Authors: Fan Wang, Tom ZieglerAbstract:In this study we have introduced a formulation of time-dependent density functional theory (TDDFT) based on a noncollinear Exchange-Correlation Potential. This formulation is a generalization of conventional TDDFT. The form of this formulation is exactly the same as that of the conventional TDDFT for the excitation energies of transitions that do not involve spin flips. In addition, this noncollinear TDDFT formulation allows for spin-flip transitions. This feature makes it possible to resolve more fully excited state spin multiplets, while for closed-shell systems, the spin-flip transitions will result in singlet-triplet excitations and this excitation energy calculated from this formulation of TDDFT is exactly the same as that from ordinary TDDFT. This formulation is applied to the dissociation of H2 in its 1Σg+ ground state and 1Σu+ and 3Σu− excited states with 3Σu− (Ms=+1) as the reference state and the multiplets splitting of some atoms.
Neepa T. Maitra - One of the best experts on this subject based on the ideXlab platform.
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Exact Time-Dependent Exchange-Correlation Potential in Electron Scattering Processes
Physical review letters, 2017Co-Authors: Yasumitsu Suzuki, Lionel Lacombe, Kazuyuki Watanabe, Neepa T. MaitraAbstract:We identify peak and valley structures in the exact Exchange-Correlation Potential of time-dependent density functional theory that are crucial for time-resolved electron scattering in a model one-dimensional system. These structures are completely missed by adiabatic approximations that, consequently, significantly underestimate the scattering probability. A recently proposed nonadiabatic approximation is shown to correctly capture the approach of the electron to the target when the initial Kohn-Sham state is chosen judiciously, and it is more accurate than standard adiabatic functionals but ultimately fails to accurately capture reflection. These results may explain the underestimation of scattering probabilities in some recent studies on molecules and surfaces.
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Universal Dynamical Steps in the Exact Time-Dependent Exchange-Correlation Potential
Physical review letters, 2012Co-Authors: Peter Elliott, Johanna I. Fuks, Angel Rubio, Neepa T. MaitraAbstract:We show that the exact Exchange-Correlation Potential of time-dependent density-functional theory displays dynamical step structures that have a spatially nonlocal and time nonlocal dependence on the density. Using one-dimensional two-electron model systems, we illustrate these steps for a range of nonequilibrium dynamical situations relevant for modeling of photochemical or physical processes: fieldfree evolution of a nonstationary state, resonant local excitation, resonant complete charge transfer, and evolution under an arbitrary field. A lack of these steps in the usual approximations yields inaccurate dynamics, for example, predicting faster dynamics and incomplete charge transfer.
Thomas W Keal - One of the best experts on this subject based on the ideXlab platform.
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The Exchange-Correlation Potential in Kohn–Sham nuclear magnetic resonance shielding calculations
The Journal of Chemical Physics, 2003Co-Authors: Thomas W Keal, David J. TozerAbstract:A simple gradient correction to the local density approximation functional is proposed, which improves the structure of the Exchange-Correlation Potential. The optimized generalized gradient approximation (GGA) functional provides uncoupled isotropic and anisotropic nuclear magnetic resonance shielding constants that are 2–3 times more accurate than those of commonly used GGAs, for a series of challenging molecules involving first- and second-row atoms; the results are competitive with those of ab initio wave function methods. A correlation is observed between the lowest occupied-virtual eigenvalue difference and the shielding accuracy. Magnetizabilities are also improved. The performance of the functional for structural and energetic predictions is investigated. These properties can be improved by relaxing the uniform electron gas condition, with no degradation in shielding quality. Atomization energies, ionization Potentials, and molecular bond lengths are then comparable to those of other GGA functiona...
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the exchange correlation Potential in kohn sham nuclear magnetic resonance shielding calculations
Journal of Chemical Physics, 2003Co-Authors: Thomas W Keal, David J. TozerAbstract:A simple gradient correction to the local density approximation functional is proposed, which improves the structure of the Exchange-Correlation Potential. The optimized generalized gradient approximation (GGA) functional provides uncoupled isotropic and anisotropic nuclear magnetic resonance shielding constants that are 2–3 times more accurate than those of commonly used GGAs, for a series of challenging molecules involving first- and second-row atoms; the results are competitive with those of ab initio wave function methods. A correlation is observed between the lowest occupied-virtual eigenvalue difference and the shielding accuracy. Magnetizabilities are also improved. The performance of the functional for structural and energetic predictions is investigated. These properties can be improved by relaxing the uniform electron gas condition, with no degradation in shielding quality. Atomization energies, ionization Potentials, and molecular bond lengths are then comparable to those of other GGA functiona...
