The n-electron valence state perturbation theory (NEVPT) belongs to the family of multireference perturbation theories. Starting from a CAS-CI zero order calculation, all the contracted double excitations are generated and a zero order Hamiltonian is built making use of Dyall's model Hamiltonian where all the bielectronic interactions between the active electrons are taken into account. The first order correction to the wave function is built as a summation of multireference functions each of which has an energy corresponding to a well defined physical process (e.g. an ionization involving the active electrons). The NEVPT approach presents some desirable characteristics such as a) invariance under orbital rotation in each of the three orbital classes (core, active and virtual), b) strict separability (size consistence) with respect to molecular dissociation, c) absence of intruder states. The theory can be applied to any solution of a CAS-CI calculation and is therefore well suited to the treatment of electronically excited states. The theory can be formulated either in a state specific approach or in a quasi-degenerate formalism. The latter is well suited in cases where the mixing of the configurations in the zero order wave function is poorly described in the variational procedure, as can occur in avoided crossings between ionic and covalent states or in excited states of mixed valence-Rydberg character. The theory is illustrated through a few significant test calculations.
The treatment of electronically excited states with multireference perturbation theory
CIMIRAGLIA, Renzo
2005
Abstract
The n-electron valence state perturbation theory (NEVPT) belongs to the family of multireference perturbation theories. Starting from a CAS-CI zero order calculation, all the contracted double excitations are generated and a zero order Hamiltonian is built making use of Dyall's model Hamiltonian where all the bielectronic interactions between the active electrons are taken into account. The first order correction to the wave function is built as a summation of multireference functions each of which has an energy corresponding to a well defined physical process (e.g. an ionization involving the active electrons). The NEVPT approach presents some desirable characteristics such as a) invariance under orbital rotation in each of the three orbital classes (core, active and virtual), b) strict separability (size consistence) with respect to molecular dissociation, c) absence of intruder states. The theory can be applied to any solution of a CAS-CI calculation and is therefore well suited to the treatment of electronically excited states. The theory can be formulated either in a state specific approach or in a quasi-degenerate formalism. The latter is well suited in cases where the mixing of the configurations in the zero order wave function is poorly described in the variational procedure, as can occur in avoided crossings between ionic and covalent states or in excited states of mixed valence-Rydberg character. The theory is illustrated through a few significant test calculations.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
