##

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:

- Strict separability (size consistence): the energy of a
collection of non-interacting systems equals the sum of the
energies of the isolated systems
- Absence of intruder states: the zero-order energies associated
to the functions of the outer space are well separated from the
zero-order energy of the state being studied, thus avoiding
divergences in the perturbation summation
- The first order correction to the wavefunction is an
eigenfunction of the spin operators and
- Electronically excited states are dealt with at the same level
of accuracy as the ground state
- NEVPT2 energies are invariant under a unitary transformation of
the active orbitals. Furthermore, the choice of canonical orbitals
for the core and virtual orbitals (the default choice) ensure that
the results coincide with those of an enlarged version of the theory
fully invariant under rotations in the core and virtual orbital
spaces, respectively
- NEVPT2 coincides with MP2 in the case of a HF wave function

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.

molpro@molpro.net 2019-03-20