  Simulations of laser-induced electron dynamics in a molecular system are
performed using time-dependent (TD) equation-of-motion (EOM) coupled-cluster
(CC) theory. The target system has been chosen to highlight potential
shortcomings of truncated TD-EOM-CC methods [represented in this work by
TD-EOM-CC with single and double excitations (TD-EOM-CCSD)], where unphysical
spectroscopic features can emerge. Specifically, we explore driven resonant
electronic excitations in magnesium fluoride in the proximity of an avoided
crossing. Near the avoided crossing, the CCSD similarity-transformed
Hamiltonian is defective, meaning that it has complex eigenvalues, and
oscillator strengths may take on negative values. When an external field is
applied to drive transitions to states exhibiting these traits, unphysical
dynamics are observed. For example, the stationary states that make up the
time-dependent state acquire populations that can be negative, exceed one, or
even be complex-valued.

Chemical Physics