  We report successful implementation of the time-dependent second-order
many-body perturbation theory using optimized orthonormal orbital functions
called time-dependent optimized second-order many-body perturbation theory
[TD-OMP2] to reach out to relatively larger chemical systems for the study of
intense-laser-driven multielectron dynamics. We apply this method to
strong-field ionization and high-order harmonic generation (HHG) of Ar. The
calculation results are benchmarked against ab initio time-dependent
complete-active-space self-consistent field (TD-CASSCF), time-dependent
optimized coupled-cluster double (TD-OCCD), and time-dependent Hartree-Fock
(TDHF) methods, as well as a single active electron (SAE) model to explore the
role of electron correlation.
