  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.
