Natural orbital analysis of ultrafast multielectron dynamics of molecules

The ionization dynamics of molecules in intense laser fields is investigated by using a time-dependent multiconfiguration theory for propagating the many-electron wave function in a grid space. We use the natural orbitals obtained from the many-electron wave function, i.e., the molecular orbitals obtained by diagonalizing the one-particle electron density matrix, to analyze the ionization process.We eliminate the ionizing portions of orbitals reaching the grid boundaries set far away from the nuclei; the occupation numbers of natural orbitals decrease due to ionization. The ionization probabilities of individual natural orbitals can be obtained from the accumulated reductions in occupation numbers. We also propose a new definition of molecular orbital energy in order to investigate the energetics of natural orbitals. It is shown that when energies are assigned to natural orbitals {φ_j(t)} as chemical potentials {ε_j(t)}, one can quantify a correction to the total electronic energy that represents electron correlation; that is, time-dependent correlation energy is introduced. Our attempt is illustrated by numerical results on the time-dependence of the spatial density and chemical potential for a H2 molecule interacting with an intense, near-infrared laser field. We compared the energy ζ_j(t) supplied by the applied field with the net energy gain Δε_j(t) in the chemical potential for φ_j(t) and found that energy accepting orbitals of Δε_j(t) > ζ_j(t) exhibit high ionization efficiency.