  We present the numerical implementation of the time-dependent
complete-active-space self-consistent-field (TD-CASSCF) method [Phys. Rev. A,
88, 023402 (2013)] for atoms driven by a strong linearly polarized laser pulse.
The present implementation treats the problem in its full dimensionality and
introduces a gauge-invariant frozen-core approximation, an efficient evaluation
of the Coulomb mean field scaling linearly with the number of basis functions,
and a split-operator method specifically designed for stable propagation of
stiff spatial derivative operators. We apply this method to high-harmonic
generation in helium, beryllium, and neon and explore the role of electron
correlations.
