  We present the time-dependent complete-active-space self-consistent-field
(TD-CASSCF) method to simulate multielectron dynamics in ultrafast intense
laser fields from the first principles. While based on multiconfiguration
expansion, it divides the orbital space into frozen-core (tightly bound
electrons with no response to the field), dynamical-core (electrons tightly
bound but responding to the field), and active (fully correlated to describe
highly excited and ejected electrons) orbital subspaces. The subspace
decomposition can be done flexibly, conforming to phenomena under investigation
and desired accuracy. The method is gauge invariant and size extensive.
Infinite-range exterior complex scaling in addition to mask-function boundary
is adopted as an efficient absorbing boundary. We show numerical examples and
illustrate how to extract relevant physical quantities such as ionization
yield, high-harmonic spectrum, and photoelectron spectrum from our
full-dimensional implementation for atoms. The TD-CASSCF method will open a way
to the ab initio simulation study of ultrafast intense laser science in
realistic atoms and molecules.
