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

Abstract

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 completeactive-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.

Perturbative and nonperturbative photoionization of H2 and H2O using the molecular R<

“…The enormous complexity of modeling dynamics of continuum molecular states embedded in an external field has led to development of several different ab initio computational approaches, each with their own strengths and weaknesses, thus providing complementary insights into the problem. This includes the haCC approach [6,7], the B-spline algebraic diagrammatic construction (ADC) code [8], the Spanner-Patchkovskii method [9], the multiconfigurational strong-field approximation with Gaussian nuclear wave packets approach (MC-SFA-GWP) [10] capable of describing coupled electronic-nuclear dynamics, the recent extension of the Xchem package [11] to calculations of field ionization of small molecules, the time-dependent multiconfiguration self-consistent-field method based on occupation-restricted multiple-active-space model (TD-ORMAS) [12], the multiconfiguration time-dependent Hartree (MCTDH) method [13,14], time-dependent coupled-cluster (TDCC) method [15][16][17], the TD close-coupling method [18], and others [19].…”

Study of laser-driven multielectron dynamics of Ne atom using time-dependent optimised second-order many-body perturbation theory

“…for general active orbitals (union of hole and particle). It should be noted that, as an orbital-optimised theory, the number of active orbitals is less than the number of the full set of the orbitals {ψ μ } in general [37,41,[46][47][48].…”

“…Recently, to further extend the applicability to heavier atoms and larger molecules interacting with intense laser fields, we have implemented approximate methods within the TD-OCC framework without losing the size-extensivity criteria [47,48]. In Ref.…”

“…In Ref. [47], we introduced a method designated as TD-OCEPA0, which is based on the simplest version of the coupled-electron pair approximation [49]. We have also implemented an approximate method in the TD-OCC framework, based on the many-body perturbation expansion of the coupled-cluster effective Hamiltonian [48].…”

“…It is size-extensive, gauge-invariant, and has a lower scaling of the computational cost [O(N 5 ) where N is the number of active orbitals] than the TD-OCC method with double excitations (TD-OCCD) having a scaling of O(N 6 ). For both the TD-OMP2 and TD-OCEPA0 [47,48], one need not solve for de-excitation amplitudes since they are the complex conjugate of the excitation amplitudes, which further leads to a significant reduction in the computational cost.…”