Vibrationally Resolved Coupled Cluster X-Ray Absorption Spectra from Vibrational Configuration Interaction Anharmonic Calculations

TL;DR

This paper presents a method to compute vibrationally resolved X-ray absorption spectra using anharmonic vibrational configuration interaction calculations, incorporating advanced coupled cluster methods and an innovative approach to include pair-mode coupling.

Contribution

It introduces a novel approach combining anharmonic vibrational calculations with coupled cluster potential energy surfaces and pair-mode coupling for polyatomic molecules.

Findings

Accurate vibrationally resolved spectra for small molecules.

Implementation of pair-mode coupling improves spectral predictions.

Use of ADGA scheme optimizes energy calculation points.

Abstract

Vibrationally resolved near-edge x-ray absorption spectra at the K-edge for a number of small molecules have been computed from anharmonic vibrational configuration interaction calculations of the Franck-Condon factors. The potential energy surfaces for ground and core-excited states were obtained at the core-valence separated CC2, CCSD, CCSDR(3), and CC3 levels of theory, employing the Adaptive Density-Guided Approach (ADGA) scheme to select the single points at which to perform the energy calculations. We put forward an initial attempt to include pair-mode coupling terms to describe the potential of polyatomic molecules