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** Next:** 23 MØLLER PLESSET PERTURBATION
** Up:** 22 NEVPT2 calculations
** Previous:** 22.1 General considerations
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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 () excited state of formaldehyde are calculated.

form_nevpt2.com

molpro@molpro.net 2018-04-21