Linear Absorption Spectra from Explicitly Time-Dependent Equation-of-Motion Coupled-Cluster Theory

We report an explicitly time-dependent approach to the generation of linear absorption spectra for molecular systems within the framework of equation-of-motion (EOM) coupled-cluster (CC) theory. While most time-dependent CC approaches consider the perturbation and time evolution of a CC wave function, the present work considers the time-evolution of a CC dipole function. The dipole function formalism introduces no approximations and requires the evolution of only one time-dependent quantity, either the left or right dipole function. This time-dependent framework can be used to compute linear absorption spectra for molecules with a high density of states over a broad spectral range, a case for which conventional frequency domain computations may become impractical. We validate the approach by comparing absorption spectra for small molecules computed at EOM second order approximate CC (CC2) and time-dependent EOM-CC2 (TD-EOM-CC2) levels of theory. TD-EOM-CC2 computations are also used to predict extreme ultraviolet absorption spectra for third-row ions that are in reasonable agreement with experiment.