Quasi-complete active space

Extension of reference wavefunctions—quasi-degenerate perturbation theory with quasi-complete active space self-consistent field reference functions (QCAS-QDPT) [35]... 

We have proposed an MC-SCF method with a quasi-complete active space (QCAS), i.e. a QCAS-SCF method. In the MC-SCF method, we partition orbitals into core, active, and virtual, then construct the Cl space by distributing active electrons among the active orbitals. Let us further divide the active electron and orbital sets into N sub-sets and fix... 

A second order MRPT for the restricted active space (RAS) and a general active space was proposed [63]. More recently another theory has been developed [64] using a QCAS (Quasi-Complete Active Space). These theories, unlike in Ref. [60], are not rigorously size-extensive. 

The studies so far made for this ESHAT employed the ab initio configuration interaction (Cl) and/or complete active space SCF (CAS-SCF) methods and examined only the two lowest coupled potential energy surfaces (and nuclear wavepacket dynamics simplified [228]). However, because of the fact that the relevant excited states are highly quasi-degenerate in reality, and in view of the complicated nature of electron dynamics associated with proton transfer as described above, it is worthwhile to re-examine the mechanism from the nonadiabatic electron wavepacket dynamics. 

QDMBPT = quasi-degenerate many-body perturbation theory, (POL-) Cl = (polarization-) configuration interaction, MC SCF = multiconfiguration SCF, CAS SCF = complete active space SCF, MRD Cl = multireference double-excitation Cl, UHF = unrestricted Hartree-Fock, UMP2 = unrestricted Moller-Plesset perturbation method of second order. - An analytical, combined polynomial-exponential form for E(x) with x = (r-rg)/r and a figure for x= —0.98 to +0.04. 

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