Time-dependent multiconfiguration theory and its application to ultrafast electronic dynamics of molecules in an intense laser field

We outlined a time-dependent multiconfiguration theory to describe electronic dynamics of molecules, where the many-electron wave function at time t, ?(t), is expanded in terms of different electron configurations ?I (t) composed of time-dependent one-electron orbitals (spin-orbitals) as ?(t) = PI CI (t)?I (t). The equations of motion (EOMs) for spin-orbitals in coordinate representation are derived together with the EOMs for configuration interaction coefficients CI (t). As an example of application to molecules, we presented the results of investigation of the ionization dynamics of H2 interacting with a near-infrared intense laser filed. By extending the concept of Hartree-Fock orbital energy to multiconfiguration theory, we newly introduced the "molecular orbital energies" of natural spin-orbitals (NSOs) {j} of a many-electron system and defined the orbital potentials j(t) and correlation energies V c j (t) of NSOs. The total energy E(t) is decomposed into individual components as E(t) = Pj ?j(t)j(t) as in thermodynamics, where ?j(t) are the occupation numbers of {j}. We proved that this type of partition of the total energy is interpreted as the time-dependent chemical potential for the two-electron system. The newly defined correlation energy V c j (t) associated with the j th NSO, involved in j(t), reflects dynamical electron correlations on the attosecond timescale. We also compared the energy (t) directly supplied by the applied field with the net energy gain δj (t) for respective natural orbitals. The responses of natural orbitals can be classified into three: δj (t) = (t) (spectator orbital); δj(t) < (t) (energy donor orbital); and δj(t) > (t) (energy acceptor orbital). We found that ionization of H2 most efficiently occurs from a time-developing energy acceptor NSO 2?g for the case of the present applied field. We concluded that energy acceptor natural orbitals play a key role in ionization processes.