  The time-dependent multiconfiguration self-consistent-field method based on
the occupation-restricted multiple active space model is proposed (TD-ORMAS)
for multielectron dynamics in intense laser fields. Extending the previously
proposed time-dependent complete-active-space self-consistent-field method
[TD-CASSCF; Phys. Rev. A, {\bf 88}, 023402 (2013)], which divides the occupied
orbitals into core and active orbitals, the TD-ORMAS method {\it further}
subdivides the active orbitals into an arbitrary number of subgroups, and poses
the {\it occupation restriction} by giving the minimum and maximum number of
electrons distributed in each subgroup. This enables highly flexible
construction of the configuration interaction (CI) space, allowing a
large-active-space simulation of dynamics, e.g., the core excitation or
ionization. The equations of motion both for CI coefficients and spatial
orbitals are derived based on the time-dependent variational principle, and an
efficient algorithm is proposed to solve for the orbital time derivatives.
In-depth descriptions of the computational implementation are given in a
readily programmable manner. The numerical application to the one-dimensional
lithium hydride cluster models demonstrates that the high flexibility of the
TD-ORMAS framework allows for the cost-effective simulations of multielectron
dynamics, by exploiting systematic series of approximations to the TD-CASSCF
method.
