The SSG description of molecular electronic structure can be improved
by perturbative description of missing inter-geminal correlation
effects. We have implemented Epstein-Nesbet form of perturbation
theory
318
Phys. Rev.
(1926),
28,
pp. 695.
Link
,
858
Proc. Roy. Soc. Ser. A
(1955),
230,
pp. 312.
Link
that permits a balanced description of one- and
two-electron contributions to excited states’ energies in SSG model.
This form of perturbation theory is especially accurate for
calculation of weak intermolecular forces. Also, two-electron
integrals are included in the reference
Hamiltonian in addition to intra-geminal
integrals that are needed for reference wave function to be an
eigenfunction of the reference Hamiltonian.
1011
J. Chem. Phys.
(2004),
120,
pp. 10385.
Link
All perturbative contributions to the SSG(EN2) energy (second-order Epstein-Nesbet perturbation theory of SSG wave function) are analyzed in terms of largest numerators, smallest denominators, and total energy contributions by the type of excitation. All excited states are subdivided into dispersion-like with correlated excitation within one geminal coupled to the excitation within another geminal, single, and double electron charge transfer. This analysis permits careful assessment of the quality of SSG reference wave function. Formally, the SSG(EN2) correction can be applied both to RSSG and USSG wave functions. Experience shows that molecules with broken or nearly broken bonds may have divergent RSSG(EN2) corrections. USSG(EN2) theory is balanced, with largest perturbative corrections to the wave function rarely exceeding 0.1 in magnitude.
SSG
SSG
Controls the calculation of the SSG wave function.
TYPE:
INTEGER
DEFAULT:
0
OPTIONS:
0
Do not compute the SSG wave function
1
Do compute the SSG wave function
RECOMMENDATION:
See also the UNRESTRICTED and DIIS_SUBSPACE_SIZE $rem
variables.