Iterative CASCI-DFT for excited states

摘要

As in the time-independent (TI) Kohn-Sham (KS) density functional theory (DFT), the time-dependent (TD) version of KS-DFT with the adiabatic approximation has successfully applied for the excitation spectra. Since the prescriptions for electronic correlation are similar among time-independent and time-dependent DFT, the TD-DFT description using an usual adiabatic approximation also breaks down for the states and the systems where KS-DFT breaks down. The electron correlations that both of TI and TD-DFT do not work for are explicit two-(and higher order) excitations in the context of wavefunction theory (WFT). In contrast, the so-called dynamical correlation effects can be covered well by both of them in implicit ways.rnAs for the ground state, the new class of the electronic structure theory combining multireference (MR) WFT has attracted much attention. The developers intend to make full use of the capability of KS-DFT for the electron correlation effects besides a MR characters. Most of the pioneering redevelopers of the MR-DFT devoted their efforts to construct a DFT correction scheme for a MR-wavefunction energy. In contrast, some of them go towards an effective MR method with involving the DFT correction, where we can obtain not only the energy-related properties, but also any properties at a high-qualitative level including both nondynamical and dynamical correlation corrections. We also proposed the iterative complete-active-space configuration interaction-DFT (ICASCI-DFT) approach, of which the system of equations is given by the effective CASCI equation with involving the DFT correlation potential. In this study, we present an application of ICASCI-DFT approach for low-lying excited states of simple molecules.