MOLPRO

22 NEVPT2 CALCULATIONS 176
IHINT=0
IHINT=1
IHINT=2
NOREF=1
NOREF=0
IMP3=2
(Default). Only one-electron integrals are used in the zerothorder
Hamiltonian for all interactions.
The all-internal two-electron integrals are used in the zerothorder
Hamiltonian for the internal-internal and single-single interactions.
The all-internal two-electron integrals in the zeroth-order Hamiltonian
are used for the internal-internal, single-single, and pairpair
interactions. Using IHINT=2 and IDFIA=1 corresponds
to Dyall’s CAS/A method for the case that CASSCF references
with no closed-shells (inactive orbitals) are used. Note that
this requires more CPU time than a standard CASPT2 calculation.
Moreover, convergence of the CAS/A method is often
slow (denominator shifts specified on a SHIFT card may be
helpful in such cases). In general, we do not recommend the
use of IHINT with nonzero values.
(Default). Interactions between reference configurations and
singles are omitted.
Interactions between reference configurations and singles are
included. This causes a relaxation of the reference coefficients
but may lead to intruder-state problems.
After CASPT2 do variational CI using all internal configurations
and the first-order wavefunctions of all states as a basis.
In this case the second-order energy will correspond to the variational
energy, and the third-order energy approximately to a
Davidson-corrected energy. This is useful in excited state calculations
with near-degeneracy situations.
22 NEVPT2 calculations
Reference literature:
C. Angeli, R. Cimiraglia, S. Evangelisti, T. Leininger and J. P. Malrieu, J. Chem. Phys.,
114, 10252, (2001)
C. Angeli, R. Cimiraglia and J. P. Malrieu, J. Chem. Phys., 117, 9138, (2002)
C. Angeli, M. Pastore and R. Cimiraglia, Theor. Chem. Acc., 117, 743 (2007)
All publications resulting from use of this program must acknowledge the above.
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: