Equation-of-motion MLCCSD and CCSD-in-HF oscillator strengths and their application to core excitations

TL;DR

This paper introduces an efficient equation-of-motion oscillator strength implementation for the MLCCSD model, combining active space restrictions and Cholesky-PAO partitioning to accurately compute core excitation spectra in complex molecules.

Contribution

The paper develops a novel MLCCSD implementation with active space restrictions and Cholesky-PAO partitioning, enabling efficient core excitation calculations.

Findings

MLCCSD accurately reproduces core excitation spectra.

Cholesky-PAO partitioning scales as O(N^3).

Comparison shows MLCCSD and CCSD-in-HF are consistent for adenosine and ATP.

Abstract

We present an implementation of equation-of-motion oscillator strengths for the multilevel CCSD (MLCCSD) model where CCS is used as the lower level method (CCS/CCSD). In this model, the double excitations of the cluster operator are restricted to an active orbital space, whereas the single excitations are unrestricted. Calculated nitrogen K-edge spectra of adenosine, adenosine triphosphate (ATP), and an ATP-water system are used to demonstrate the performance of the model. Projected atomic orbitals (PAOs) are used to partition the virtual space into active and inactive orbital sets. Cholesky decomposition of the Hartree-Fock density is used to partition the occupied orbitals. This Cholesky-PAO partitioning is cheap, scaling as $\mathcal{O}(N^3)$, and is suitable for the calculation of core excitations which are localized in character. By restricting the single excitations of the cluster operator to the active space, as well as the double excitations, the CCSD-in-HF model is obtained. A comparison of the two models---MLCCSD and CCSD-in-HF---is presented for the core excitation spectra of the adenosine and ATP systems.