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22.1 General considerations

NEVPT2 is a form of second-order multireference perturbation theory which can be applied to CAS-SCF wavefunctions or, more generally, to CAS-CI wavefunctions. The term NEVPT is an acronym for ``n-electron valence state perturbation theory''. While we refer the reader to the pertinent literature (see above), we limit ourselves to recalling here that the most relevant feature of NEVPT2 consists in that the first order correction to the wave function is expanded over a set of properly chosen multireference functions which correctly take into consideration the two-electron interactions occurring among the active electrons. Among the properties ensured by NEVPT2 we quote:

NEVPT2 has been implemented in two variants both of which are present in MOLPRO, these are the strongly contracted (SC) and the partially contracted (PC) variants. The two variants differ by the number of perturber functions employed in the perturbation summation. The PC-NEVPT2 uses a richer function space and is in general more accurate than the SC-NEVPT2. The results of SC-NEVPT2 and PC-NEVPT2 are anyway usually very close to one another.



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