Time-dependent multiconfiguration theory for ultrafast electronic dynamics of molecules in an intense laser field: Electron correlation and energy redistribution among natural orbitals

We propose a new definition of molecular orbital energy in order to investigate the energetics of constituent molecular orbitals in the many-electron wave function calculated based on time-dependent multi-configuration theory. It is shown that when energies are assigned to natural orbitals by a similar manner to that used in the Hartree-Fock theory, we can quantify a correction energy to the total electronic energy that represents electron correlation, and thus we can evaluate the time-dependence of the correlation energy. Our attempt is illustrated by numerical results on the time-dependence of the spatial density of the correlation energy and the orbital energies for a H(2) molecule interacting with an intense, near-infrared laser field. We compared the energy zeta(j)(t) supplied by the applied field with the net energy gain Lambda(epsilon) over bar (j)(t) for respective natural orbitals phi(j)(t), phi(j) and found that the natural orbitals with Lambda(epsilon) over bar (j)(t) > zeta(j)(t) play a key role in the ionization process. (C) 2009 Elsevier BV. All rights reserved.