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22.2 Input description

NEVPT2 must follow a CAS-SCF or CAS-CI calculation. The command

NEVPT2,options
has to be specified to carry out a second-order perturbation calculation. NEVPT2 is part of the MRCI program and uses the options of the latter. Of particular relevance are the options CORE, CLOSED, OCC, WF and STATE of the MRCI program. There is, at the moment, only one option specific to NEVPT2 which can be provided by the user:

THRNEVPT2

The threshold to discard small coefficients in the CAS wavefunction (default = 0.0),

The present implementation of NEVPT2 is state-specific, i.e. the perturbation theory can only be applied to a single state. The multi-state (or quasi-degenerate) version of NEVPT2 will be implemented in MOLPRO in the near future.

An example is provided where the energies of the ground state and of the first $^1A_2$ ($n \to \pi^*$) excited state of formaldehyde are calculated.

form_nevpt2.com