Description of electronic dynamics of molecules in a strong laser field by a time-dependent molecular orbital approximation

We propose a theory to calculate the electronic dynamics (time-dependent electronic wave function) of molecules in a strong laser field based on a time-dependent natural orbitals expansion. The theory is applicable for general N-electron system. Unitary transformations among natural orbitals are performed in the time evolution, which are governed by Dirac-Frenkel lime-dependent variational principal. The theory is equivalent to the full-CI approach for the full use of the electron configuration functions for a given set of the atomic orbitals, and is reduced to the time-dependent Hartree-Fock theory for a single configuration function. The numerical results obtained by the first application of the present theory to a hydrogen molecule are consistent with the above expectations for the performance in both the imaginary- and the real-time propagations.