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Physical Review A

Influence of the noninteracting density response function on the exchange-only kernel in time-dependent density-functional theory

Norman Henry March, Thomas A. Niehaus, Sáandor Suhai

Published Oct 6, 2006• 1 citation
Publisher

Abstract

A formally exact expression for the interaction density response function $\ensuremath{\chi}(\mathbf{r},{\mathbf{r}}^{\ensuremath{'}},\ensuremath{\omega})$ exists in terms of (i) its noninteracting counterpart ${\ensuremath{\chi}}_{0}(\mathbf{r},{\mathbf{r}}^{\ensuremath{'}},\ensuremath{\omega})$ and (ii) an exchange $(x)$-correlation $(c)$ kernel ${f}_{xc}(\mathbf{r},{\mathbf{r}}^{\ensuremath{'}},\ensuremath{\omega})$. In the absence of a first-principles theory for the $\ensuremath{\omega}$ dependence of ${f}_{xc}$, the adiabatic approximation is most frequently made in this term, to construct a workable time-dependent density-functional theory. In the present study, a proposal is put forward to avoid the adiabatic approximation by working in the exchange-only limit in which ${f}_{xc}$ is set equal to ${f}_{x}(\mathbf{r},{\mathbf{r}}^{\ensuremath{'}},\ensuremath{\omega})$. We then refer to a result for the exchange energy given by Pines and Nozieres to motivate the assumption that ${f}_{x}={f}_{x}[\mathrm{Im}\phantom{\rule{0.2em}{0ex}}{\ensuremath{\chi}}_{0}(\mathbf{r},{\mathbf{r}}^{\ensuremath{'}},\ensuremath{\omega})]$. The essential proposal here is therefore that the integral equation to be solved for the interacting density response function $\ensuremath{\chi}$ is in the exchange-only case characterized entirely by the noninteracting response function ${\ensuremath{\chi}}_{0}$.

DOI
10.1103/physreva.74.044502

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We consider an analytically solvable model of two interacting electrons that allows for the calculation of the exact exchange-correlation kernel of time-dependent density functional theory. This kernel, as well as the corresponding density response function, is studied in the limit of large repulsive interactions between the electrons and we give analytical results for these quantities as an asymptotic expansion in powers of the square root of the interaction strength. We find that in the strong interaction limit the three leading terms in the expansion of the kernel act instantaneously while memory terms only appear in the next orders. We further derive an alternative expansion for the kernel in the strong interaction limit on the basis of the theory developed previously [Phys. Chem. Chem. Phys. 18, 21092 (2016)] using the formalism of strictly correlated electrons in the adiabatic approximation. We find that the first two leading terms in this series, corresponding to the strictly correlated limit and its zero-point vibration correction, coincide with the two leading terms of the exact expansion. We finally analyze the spatial nonlocality of these terms and show when the adiabatic approximation breaks down. The ability to reproduce the exact kernel in the strong interaction limit indicates that the adiabatic strictly correlated electron formalism is useful for studying the density response and excitation properties of other systems with strong electronic interactions.
The interacting and the Kohn-Sham static density-density response functions for different one-dimensional two-electron singlet systems are reconstructed numerically. From their inverse we obtain the exact static exchange-correlation kernel. This quantity represents the adiabatically exact approximation of the frequency-dependent exchange-correlation kernel that is crucial for time-dependent linear density-response theory. We investigate its performance for nonlocal perturbations and analyze its sum rule properties. We also compute the adiabatically exact transition energies that follow from the static kernel within linear-response theory.
In time-dependent density-functional theory, an important quantity is the exchange-correlation kernel whose knowledge allows for the calculations of the excitation spectrum of electronic systems. In most applications, the exchange-correlation kernel is approximated by the adiabatic local-density approximation, ignoring the frequency dependence and nonlocality. To gain insight into the nature of the exact exchange-correlation kernel, we have considered a two-dimensional one-band Hubbard model. The calculated exact exchange-correlation kernel reveals a number of striking features. It has a weak energy dependence up to the main excitation energy. On the other hand, the exact kernel shows a strong energy dependence in energy regions, where there are many-body excitations that are not contained in the local-density approximation, even though these excitations have small weight. It is found that a static approximation can well reproduce the main excitation peaks, but the satellite structures arising from many-body interaction are unlikely to be accounted for with a static approximation.
The exact exchange-correlation (xc) kernel ${f}_{\text{xc}}(x,{x}^{\ensuremath{'}},\ensuremath{\omega})$ of linear response time-dependent density functional theory is computed over a wide range of frequencies for three canonical one-dimensional finite systems. Methods used to ensure the numerical robustness of fxc{f}_{\text{xc}}fxc​ are set out. The frequency dependence of fxc{f}_{\text{xc}}fxc​ is found to be largely due to its analytic structure, i.e., its singularities at certain frequencies, which are required in order to capture particular transitions, including those of double excitation character. However, within the frequency range of the first few interacting excitations, fxc{f}_{\text{xc}}fxc​ is approximately $\ensuremath{\omega}$ independent, meaning the exact adiabatic approximation ${f}_{\text{xc}}(\ensuremath{\omega}=0)$ remedies the failings of the local density approximation and random phase approximation for these lowest transitions. The key differences between the exact fxc{f}_{\text{xc}}fxc​ and its common approximations are analyzed, and cannot be eliminated by exploiting the limited gauge freedom in fxc{f}_{\text{xc}}fxc​. The optical spectrum benefits from using as accurate as possible an fxc{f}_{\text{xc}}fxc​ and ground-state xc potential, while maintaining exact compatibility between the two is of less importance.
A diagrammatic expansion for the dynamic exchange-correlation kernel fxc{f}_{\mathrm{xc}}fxc​ of time-dependent density functional theory is formulated. It is shown that fxc{f}_{\mathrm{xc}}fxc​ has no singularities at Kohn-Sham transition energies in every order of the perturbation theory. However, it may diverge with the system size in extended systems. This signifies that any approximate perturbative substitute for fxc{f}_{\mathrm{xc}}fxc​ requires a consistent perturbative treatment of the response function to avoid uncontrollable errors in the many-body corrections to excitations energies.
We investigate a number of formal properties of the adiabatic strictly-correlated electrons (SCE) functional, relevant for time-dependent potentials and for kernels in linear response time-dependent density functional theory. Among the former, we focus on the compliance to constraints of exact many-body theories, such as the generalised translational invariance and the zero-force theorem. Within the latter, we derive an analytical expression for the adiabatic SCE Hartree exchange-correlation kernel in one dimensional systems, and we compute it numerically for a variety of model densities. We analyse the non-local features of this kernel, particularly the ones that are relevant in tackling problems where kernels derived from local or semi-local functionals are known to fail.