Accurate core excitation and ionization energies from a state-specific coupled-cluster singles and doubles approach

Abstract

We investigate the use of orbital-optimized references in conjunction with single-reference coupled-cluster theory with single and double substitutions (CCSD) for the study of core excitations and ionizations of 18 small...

Generalization of One-Center Nonorthogonal Configuration Interaction Singles to Open-Shell Singlet Reference States: Theory and Application to Valence-Core Pump-Probe States in Acetylacetone

“…44,45 Finally, for the purpose of benchmarking core excited state energies, state-specific coupled-cluster methods relying on excited-state references have recently been studied. 46,47 Relative to the development of electronic structure methods for computing the NEXAS of closed-shell systems, the theoretical modeling of the NEXAS of open-shell radicals remains in an exploratory stage due to a collection of challenges. First, common to both closed-shell and radical systems, an electronic structure model must account for corehole relaxation to obtain reasonable accuracy.…”

Excited-State-Specific Pseudoprojected Coupled-Cluster Theory

“…Motivated in part by the limitations of linear response, there has been much recent work on excited-state-specific methods. Whether studying single-determinant wavefunctions, − configuration interaction (CI) wavefunctions, CC wavefunctions, − or DFT functionals, − the theme of locating higher energy, state-specific excited-state and open-shell roots is becoming increasingly prevalent across the field. Recognizing that, in the ground state, CCSD is a crucial stepping stone toward CCSD­(T) as well as a useful method in its own right, our focus here is to construct a CCSD-like excited-state-specific CC theory atop an excited-state mean field (ESMF) starting point − that, like HF in the ground state, has already taken care of state-specific orbital relaxations.…”

Theoretical Investigation of the X-ray Stark Effect in Small Molecules

“…Among such spectroscopic techniques, high-resolution x-ray based spectroscopies have emerged as powerful probes for determining the local structure of atoms and molecules, particularly in the soft x-ray regime (roughly 100–1000 eV). − Since x-ray absorption spectroscopy (XAS) and x-ray photoelectron spectroscopy (XPS) involve the excitation of core level electrons, these are excellent techniques for studying the local environment on a submolecular scale. The challenges and recent advances in the methods for simulating x-ray spectra are presented in a recent review article by Norman et al In this article, we are especially interested in the application of equation-of-motion coupled cluster (EOM-CC) techniques, for which many theoretical methods are being developed to increase the accuracy and efficiency of simulating XAS and XPS of molecules. − Additionally, theoretical methods are being developed to simulate other x-ray spectroscopies, such as x-ray two photon absorption, x-ray circular dichroism spectra, and resonant inelastic x-ray scattering . Further, x-ray based spectroscopies provide elemental specificity, which is not possible in vibrational or UV–vis spectroscopies.…”