Time-dependent complete-active-space self-consistent-field method for multielectron dynamics in intense laser fields

Abstract

The time-dependent complete-active-space self-consistent-field (TD-CASSCF) method for the description of multielectron dynamics in intense laser fields is presented, and a comprehensive description of the method is given. It introduces the concept of frozen-core (to model tightly bound electrons with no response to the field), dynamical-core (to model electrons tightly bound but responding to the field), and active (fully correlated to describe ionizing electrons) orbital subspaces, allowing compact yet accurate representation of ionization dynamics in many-electron systems. The classification into the subspaces can be done flexibly, according to simulated physical situations and desired accuracy, and the multiconfiguration time-dependent Hartree-Fock (MCTDHF) approach is included as a special case. To assess its performance, we apply the TD-CASSCF method to the ionization dynamics of one-dimensional lithium hydride (LiH) and LiH dimer models, and confirm that the present method closely reproduces rigorous MCTDHF results if active orbital space is chosen large enough to include appreciably ionizing electrons. The TD-CASSCF method will open a way to the first-principle theoretical study of intense-field induced ultrafast phenomena in realistic atoms and molecules.PACS numbers: 32.80. Rm, 42.65.Ky

The structure of approximate two electron wavefunctions in intense laser driven ionization dynamics

“…Note that all the methods examined in this work are gauge-invariant as discussed, e.g., in Ref. [24]. In all simulations, spatial functions are discretized on equidistant grid points with spacing ∆z = 0.4 within a simulation box −1000 < z < 1000.…”

“…See Ref. [24] for more comprehensive review of the proposed theoretical approaches including very recent developments.…”

Time-dependent generalized-active-space configuration-interaction approach to photoionization dynamics of atoms and molecules

“…A twoelectron hydrogen-like molecule is then defined by Z 1 = Z 2 = 1 and a four-electron lithium-hydride equivalent by Z 1 = 3 and Z 2 = 1. Such models are well-established in the literature, e.g., [11,12,26,73,84]. We point out that for x c ≫ R, the choice of the grid reference, i.e.…”

“…This makes the method infeasible for systems having more than a few electrons interacting with a strong field. The TD complete-active-space self-consistent-field method (TD-CASCF) [26], and the more general TD restricted-active space SCF (TD-RASSCF) [27][28][29] cure this scaling by imposing restrictions on the active orbital spaces, while keeping the attractive SCF notion of the MCTDHF approach, i.e., the orbitals are time-dependent and optimally updated in each time-step.…”

Ultrafast population transfer to excited valence levels of a molecule driven by x-ray pulses

“…The multiconfiguration time-dependent Hartree-Fock (MCTDHF) algorithm [24][25][26][27][28][29][30][31][32] is an adaptive method for calculating nonpertubative electronic dynamics in molecules. In a recently described numerical implementation [33,34] that we use here, all electrons are active, all orbitals are time-dependent, and any number of them may be ionized.…”