Time dependent complete active space self consistent field method for atoms: Application to high order harmonic generation
We present a 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.