  We present a cost-effective treatment of the triple excitation amplitudes in
the time-dependent optimized coupled-cluster (TD-OCC) framework called
TD-OCCDT(4) for studying intense laser-driven multielectron dynamics. It
considers triple excitation amplitudes correct up to fourth-order in many-body
perturbation theory and achieves a computational scaling of O(N7), with N being
the number of active orbital functions. This method is applied to the electron
dynamics in Ne and Ar atoms exposed to an intense near-infrared laser pulse
with various intensities. We benchmark our results against the time-dependent
complete-active-space self-consistent field (TD-CASSCF), time-dependent
optimized coupled-cluster with double and triple excitations (TD-OCCDT),
time-dependent optimized coupled-cluster with double excitations (TD-OCCD), and
the time-dependent Hartree-Fock (TDHF) methods to understand how this
approximate scheme performs in describing nonperturbatively nonlinear
phenomena, such as field-induced ionization and high-harmonic generation. We
find that the TD-OCCDT(4) method performs equally well as the TD-OCCDT method,
almost perfectly reproducing the results of fully-correlated TD-CASSCF with a
more favorable computational scaling.
