MOLPRO

22 NEVPT2 CALCULATIONS 177
• 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 S 2
and S z
• 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.
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 1 A 2 (n → π ∗ )
excited state of formaldehyde are calculated.