Quasi-degenerate perturbation theory QDPT

Electron correlation was treated by the CIPSI multi-reference perturbation algorithm ([24,25] and refs, therein). The Quasi Degenerate Perturbation Theory (QDPT) version of the method was employed, with symmetrisation of the effective hamiltonian [26], and the Maller-Plesset baricentric (MPB) partition of the C.I. hamiltonian. 

Our multireference M0Uer-Plesset (MRMP) perturbation method [1-4] and MC-QDPT quasi-degenerate perturbation theory (QDPT) with multiconfiguration self-consistent field reference functions (MC-QDPT) [5,6] are perturbation methods of such a type. Using these perturbation methods, we have clarified electronic stmctures of various systems and demonstrated that they are powerful tools for investigating excitation spectra and potential energy surfaces of chemical reactions [7-10]. In the present section, we review these multireference perturbation methods as well as a method for interpreting the electronic structure in terms of valence-bond resonance structure based on the CASSCF wavefunction. 

Eq.(9) in a power expansion, thus going back to a Rayleigh-Schrddinger version of the quasi-degenerate perturbation theory (QDPT (ii) use a monoelectronic definition of Ho = (Moller-Plesset definition of the... 

Multiconfigurational quasi-degenerate perturbation theory (MC-QDPT) [5,6] We have also proposed a multistate multireference perturbation theory, the QDPT with MCSCF reference functions (MC-QDPT). In this PT, state-averaged CASSCF is first performed to set reference functions, and then an elfective Hamiltonian is constructed, which is finally diagonalized to obtain the energies of interest. 

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

DIM = diatomics-in-molecules MRDCI = multi-reference single and double excitation configuration interaction QDPT = quasi-degenerate perturbation theory. 

Given the remarkable progress in many-body theories, accurate descriptions of electron correlation in molecular systems of variable near-degeneracies are still challenging and remain an active area of research. One framework that has provided not only accurate results but also qualitative insight is effective Hamiltonian (EH), based on which various multi-reference (MR) or quasi-degenerate (QD) perturbation theories (PT) have been proposed [ 1-26] in the past (see Refs. [27] and [28] for careful comparisons). Yet, the premises of MRPTs and QDPTs that electron correlation can be separated into static and dynamic components and then that they can respectively be treated variationally and per-turbatively are not always justified. As a consequence, low-order MRPTs and QDPTs usually depend strongly on the... 

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