Alex P. Gaiduk

How to tell when a model Kohn–Sham potential is not a functional derivative

A model exchange-correlation potential constructed with Kohn-Sham orbitals should be a functional derivative of some density functional. Several necessary conditions for a functional derivative are discussed including: (i) minimization of the total-energy expression by the ground-state solution of the Kohn-Sham equations, (ii) path independence of the van Leeuwen-Baerends line integral, and (iii) net zero force and zero torque on the density. A number of existing model potentials are checked for these properties and it is found that most of the potentials tested are not functional derivatives. Physical properties obtained from potentials that have no parent functionals are ambiguous and, therefore, should be interpreted with caution.

Density and Compressibility of Liquid Water and Ice from First-Principles Simulations with Hybrid Functionals

We determined the equilibrium density and compressibility of water and ice from first-principles molecular dynamics simulations using gradient-corrected (PBE) and hybrid (PBE0) functionals. Both functionals predicted the density of ice to be larger than that of water, by 15 (PBE) and 35% (PBE0). The PBE0 functional yielded a lower density of both ice and water with respect to PBE, leading to better agreement with experiment for ice but not for liquid water. Approximate inclusion of dispersion interactions on computed molecular-dynamics trajectories led to a substantial improvement of the PBE0 results for the density of liquid water, which, however, resulted to be slightly lower than that of ice.

Virial exchange energies from model exact-exchange potentials

It is shown by the example of Slaters averaged exchange potential that a poor approximation to the optimized effective potential (OEP) can yield a deceptively accurate energy via the conventional Kohn-Sham energy functional. For a trial exchange potential to be correct, its Kohn-Sham energy must coincide with the value obtained by the Levy-Perdew virial relation. Significant discrepancies between Kohn-Sham and the virial exchange energies are found for self-consistent Slater, Becke-Johnson, and effective local potentials (ELPs); their relative magnitudes are used to argue that, as approximations to the exact-exchange OEP, ELPs are the most accurate. Virial energy discrepancies vanish for Yang-Wu OEPs when the orbital and auxiliary basis sets are balanced, and remain surprisingly small for oscillatory OEPs obtained with unbalanced basis sets.

Reconstruction of Density Functionals from Kohn-Sham Potentials by Integration along Density Scaling Paths.

We demonstrate by specific examples that if a Kohn-Sham exchange-correlation potential is given explicitly in terms of the electron density and its derivatives, then one can easily reconstruct the parent density functional by evaluating analytically (or numerically with one-dimensional quadratures) the van Leeuwen-Baerends line integral (Phys. Rev. A 1995, 51, 170-178) along a path of (coordinate)-scaled densities. The choice of a density scaling path amounts to defining the gauge of the resultant exchange-correlation energy density. The well-known Levy-Perdew virial relation for exchange potentials can be viewed as an analytical line integral along the electron-number-conserving uniform density scaling path. Energies obtained from model exchange-correlation potentials should be interpreted with caution because the reconstructed density functional is unique (up to a gauge transformation) only if the model Kohn-Sham potential is a functional derivative.

Photoelectron Spectra of Aqueous Solutions from First Principles

We present a combined computational and experimental study of the photoelectron spectrum of a simple aqueous solution of NaCl. Measurements were conducted on microjets, and first-principles calculations were performed using hybrid functionals and many-body perturbation theory at the G0W0 level, starting with wave functions computed in ab initio molecular dynamics simulations. We show excellent agreement between theory and experiments for the positions of both the solute and solvent excitation energies on an absolute energy scale and for peak intensities. The best comparison was obtained using wave functions obtained with dielectric-dependent self-consistent and range-separated hybrid functionals. Our computational protocol opens the way to accurate, predictive calculations of the electronic properties of electrolytes, of interest to a variety of energy problems.

Local and Global Effects of Dissolved Sodium Chloride on the Structure of Water

Determining how the structure of water is modified by the presence of salts is instrumental to understanding the solvation of biomolecules and, in general, the role played by salts in biochemical processes. However, the extent of hydrogen bonding disruption induced by salts remains controversial. We performed extensive first-principles simulations of solutions of a simple salt (NaCl) and found that, while the cation does not significantly change the structure of water beyond the first solvation shell, the anion has a further reaching effect, modifying the hydrogen-bond network even outside its second solvation shell. We found that a distinctive fingerprint of hydrogen bonding modification is the change in polarizability of water molecules. Molecular dipole moments are instead insensitive probes of long-range modifications induced by Na+ and Cl- ions. Though noticeable, the long-range effect of Cl- is expected to be too weak to affect solubility of large biomolecules.

Removal of Basis-Set Artifacts in Kohn–Sham Potentials Recovered from Electron Densities

Kohn-Sham effective potentials recovered from Gaussian-basis-set electron densities exhibit large oscillations and asymptotic divergences not found in exact potentials and in functional derivatives of approximate density functionals. We show that the detailed structure of these oscillations and divergences is almost exclusively determined by the basis set in terms of which the reference density is expressed, and is almost independent of the density-functional or wave function method used for computing the density. Based on this observation, we propose a smoothening scheme in which most basis-set artifacts in a Kohn-Sham potential recovered from a given density are removed by subtracting the oscillation profile of the exchange-only local-density approximation potential computed in the same basis set as the reference density. The correction allows one to obtain smooth Kohn-Sham potentials from electron densities even for small Gaussian basis sets and greatly reduces discrepancies between the original (input) density and the density obtained from the reconstructed potential.

Efficient construction of exchange and correlation potentials by inverting the Kohn–Sham equations

Given a set of canonical Kohn-Sham orbitals, orbital energies, and an external potential for a many-electron system, one can invert the Kohn-Sham equations in a single step to obtain the corresponding exchange-correlation potential, vXC(r). For orbitals and orbital energies that are solutions of the Kohn-Sham equations with a multiplicative vXC(r) this procedure recovers vXC(r) (in the basis set limit), but for eigenfunctions of a non-multiplicative one-electron operator it produces an orbital-averaged potential. In particular, substitution of Hartree-Fock orbitals and eigenvalues into the Kohn-Sham inversion formula is a fast way to compute the Slater potential. In the same way, we efficiently construct orbital-averaged exchange and correlation potentials for hybrid and kinetic-energy-density-dependent functionals. We also show how the Kohn-Sham inversion approach can be used to compute functional derivatives of explicit density functionals and to approximate functional derivatives of orbital-dependent functionals.

Communication: Explicit construction of functional derivatives in potential-driven density-functional theory

We propose a method for imposing an important exact constraint on model Kohn-Sham potentials, namely, the requirement that they be functional derivatives of functionals of the electron density ρ. In particular, we show that if a model potential v(r) involves no ingredients other than ρ, ∇ρ, and ∇(2)ρ, then the necessary and sufficient condition for v(r) to be a functional derivative is ∂v/∂∇ρ=∇(∂v/∂∇(2)ρ). Integrability conditions of this type can be used to construct functional derivatives without knowing their parent functionals. This opens up possibilities for developing model exchange-correlation potentials that do not lead to unphysical effects common to existing approximations. Application of the technique is illustrated with examples.

A generalized gradient approximation for exchange derived from the model potential of van Leeuwen and Baerends

The common way to obtain energies from Kohn-Sham exchange potentials is by using the Levy-Perdew virial relation. For potentials that are not functional derivatives (i.e., nearly all model exchange potentials in existence), this approach leads to energy expressions that lack translational and rotational invariance. We propose a method for constructing potential-based energy functionals that are free from these artifacts. It relies on the same line-integration technique that gives rise to the Levy-Perdew relation, but uses density scaling instead of coordinate scaling. The method is applicable to any exchange or correlation potential that depends on the density explicitly, and correctly recovers the parent energy functional from a functional derivative. To illustrate our approach we develop a properly invariant generalized gradient approximation for exchange starting from the model potential of van Leeuwen and Baerends.