Time-Dependent Relativistic Two-Component Equation-of-Motion Coupled-Cluster for Open-Shell Systems: TD-EA/IP-EOMCC

TL;DR

This paper introduces a combined imaginary-time and real-time approach within a relativistic framework to compute absorption spectra of open-shell systems, improving accuracy and efficiency over traditional frequency domain methods.

Contribution

It develops a novel time-dependent method for calculating spectra of open-shell systems at the relativistic EOMCC level, incorporating imaginary-time propagation for initial state approximation.

Findings

Spectra closely match those from standard frequency domain methods.

Low-lying states with non-negligible overlap affect propagation length and spectral accuracy.

The method effectively handles open-shell atomic and diatomic systems.

Abstract

We present a combined imaginary-time/real-time time-dependent (TD) approach for evaluating linear absorption spectra of open-shell systems at the electron attachment (EA) and ionization potential (IP) equation-of-motion coupled-cluster (EOMCC) levels of theory and within the exact two-component relativistic framework. The absorption lineshape is given by the Fourier transform of the electric dipole autocorrelation function, which is obtained from a real-time simulation. Approximations of the lowest-energy EA- and IP-EOMCC eigenstates, which are required as initial states for the real-time simulation, are generated by propagating a Koopmans EA/IP state in imaginary time. TD-EA/IP-EOMCC linear absorption spectra of open-shell atomic (Na, K, Rb, F, Cl, and Br) and diatomic (SiH and ClO) systems closely reproduce those obtained from standard TD-EA/IP procedures carried out in the frequency domain. We find that the existence of low-lying states with non-negligible overlap with the Koopmans determinant impacts the length of the imaginary-time propagation required to obtain an initial state that produces correct absolute energies and peak height intensities in spectra extracted from the subsequent real-time TD-EA/IP-EOMCC calculations.