  We present the time-dependent restricted-active-space self-consistent field
(TD-RASSCF) theory as a new framework for the time-dependent many-electron
problem. The theory generalizes the multiconfigurational time-dependent
Hartree-Fock (MCTDHF) theory by incorporating the restricted-active-space
scheme well known in time-independent quantum chemistry. Optimization of the
orbitals as well as the expansion coefficients at each time step makes it
possible to construct the wave function accurately while using only a
relatively small number of electronic configurations. In numerical calculations
of high-order harmonic generation spectra of a one-dimensional model of atomic
beryllium interacting with a strong laser pulse, the TD-RASSCF method is
reasonably accurate while largely reducing the computational complexity. The
TD-RASSCF method has the potential to treat large atoms and molecules beyond
the capability of the MCTDHF method.
