Transition moments between excited electronic states from the Hermitian formulation of the coupled cluster quadratic response function

TL;DR

This paper presents a new coupled cluster method for calculating transition moments between excited states that ensures Hermitian symmetry and provides accurate results for alkali earth atoms, including previously unreported triplet state transitions.

Contribution

A novel expectation value formalism for coupled cluster theory that maintains Hermitian symmetry and computes transition moments between excited states more accurately.

Findings

Accurately computed transition probabilities for alkali earth atoms.

Method ensures Hermitian symmetry even with truncated excitations.

Reported new transition moments between triplet excited states.

Abstract

We introduce a new method for the computation of the transition moments between the excited electronic states based on the expectation value formalism of the coupled cluster theory [B. Jeziorski and R. Moszynski, Int. J. Quant. Chem. 48, 161 (1993)]. The working expressions of the new method solely employ the coupled cluster amplitudes. In the approximation adopted in the present paper the cluster expansion is limited to single, double, and linear triple excitations. The computed dipole transition probabilities for the singlet-singlet and triplet-triplet transitions in alkali earth atoms agree well with the available theoretical and experimental data. In contrast to the existing coupled cluster response theory, the matrix elements obtained by using our approach satisfy the Hermitian symmetry even if the excitations in the cluster operator are truncated. As a part of the numerical evidence for the new method, we report calculations of the transition moments between the excited triplet states which have not yet been reported in the literature within the coupled cluster theory. Slater-type basis sets constructed according to the correlation-consistency principle are used in our calculations.