  We report the implementation of a cost-effective approximation method within
the framework of time-dependent optimized coupled-cluster (TD-OCC) method [J.
Chem. Phys. 148, 051101 (2018)] for real-time simulations of intense
laser-driven multielectron dynamics. The method, designated as TD-OCEPA0, is a
time-dependent extension of the simplest version of the coupled-electron pair
approximation with optimized orbitals [J. Chem. Phys. 139, 054104 (2013)]. It
is size extensive, gauge invariant, and computationally much more efficient
than the TD-OCC with double excitations (TD-OCCD). We employed this method to
simulate the electron dynamics in Ne and Ar atoms exposed to intense near
infrared laser pulses with various intensities. The computed results, including
high-harmonic generation spectra and ionization yields, are compared with those
of various other methods ranging from uncorrelated time-dependent Hartree-Fock
(TDHF) to fully-correlated (within the active orbital space) time-dependent
complete-active-space self-consistent-field (TD-CASSCF). The TD-OCEPA0 results
show a good agreement with TD-CASSCF ones for moderate laser intensities. For
higher intensities, however, TD-OCEPA0 tends to overestimate the correlation
effect, as occasionally observed for CEPA0 in the ground-state correlation
energy calculations.
