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We report the implementation of a cost-effective approximation method withinthe framework of time-dependent optimized coupled-cluster (TD-OCC) method [J.Chem. Phys. 148, 051101 (2018)] for real-time simulations of intenselaser-driven multielectron dynamics. The method, designated as TD-OCEPA0, is atime-dependent extension of the simplest version of the coupled-electron pairapproximation with optimized orbitals [J. Chem. Phys. 139, 054104 (2013)]. Itis size extensive, gauge invariant, and computationally much more efficientthan the TD-OCC with double excitations (TD-OCCD). We employed this method tosimulate the electron dynamics in Ne and Ar atoms exposed to intense nearinfrared laser pulses with various intensities. The computed results, includinghigh-harmonic generation spectra and ionization yields, are compared with thoseof various other methods ranging from uncorrelated time-dependent Hartree-Fock(TDHF) to fully-correlated (within the active orbital space) time-dependentcomplete-active-space self-consistent-field (TD-CASSCF). The TD-OCEPA0 resultsshow a good agreement with TD-CASSCF ones for moderate laser intensities. Forhigher intensities, however, TD-OCEPA0 tends to overestimate the correlationeffect, as occasionally observed for CEPA0 in the ground-state correlationenergy calculations.

Time-dependent optimized coupled-cluster method for multielectron dynamics II. A coupled electron-pair approximation