Fast Time-Dependent Density Functional Theory Calculations of the X-ray Absorption Spectroscopy of Large Systems.

Nicholas A Besley

doi:10.1021/acs.jctc.6b00656

Abstract

The computational cost of calculations of K-edge X-ray absorption spectra using time-dependent density functional (TDDFT) within the Tamm-Dancoff approximation is significantly reduced through the introduction of a severe integral screening procedure that includes only integrals that involve the core s basis function of the absorbing atom(s) coupled with a reduced quality numerical quadrature for integrals associated with the exchange and correlation functionals. The memory required for the calculations is reduced through construction of the TDDFT matrix within the absorbing core orbitals excitation space and exploiting further truncation of the virtual orbital space. The resulting method, denoted fTDDFTs, leads to much faster calculations and makes the study of large systems tractable. The capability of the method is demonstrated through calculations of the X-ray absorption spectra at the carbon K-edge of chlorophyll a, C60 and C70.

Highlights

In recent years, advances in the intensity and resolution obtainable with synchrotron radiation has led to significant progress in spectroscopic techniques in the X-ray region.[1]
We focus on X-ray absorption spectroscopy (XAS), or near-edge X-ray absorption fine structure (NEXAFS), which arises from the excitation of a core electron to low-lying virtual orbitals to form a state below the ionization threshold
Many early calculations of XAS were based on the direct static exchange (STEX) method.[9−11] The approximations made in this approach include the neglect of electron correlation and the independent channel approximation

Summary

Introduction

Advances in the intensity and resolution obtainable with synchrotron radiation has led to significant progress in spectroscopic techniques in the X-ray region.[1]. For the calculation of X-ray absorption spectra it is usual to limit A to include only excitations from the relevant core orbitals.[26] Within this approach, the roots corresponding to the required core-excitations are the lowest energy roots and can be found efficiently using the Davidson procedure.[50] Since there is virtually no mixing between the excitations at a given K-edge and other excitations in the molecule, this approximation has no significant effect on the computed transition energies and oscillator strengths.

Results

Conclusion