Valence Correlation Models
The SSG, or APSSG (Antisymmetrized Product of Singlet-type Strongly Orthogonal Geminals) is a multireference model that is designed for description of molecules where traditional single reference models fail, such as bond-breaking, transition metal chemistry, cases where two oxygen atoms bond to the common atom, etc. Unlike all other models, SSG appears to capture the multireference character of any chemical system. It is both variational and size-consistent. The SSG can be considered as an extension of the Generalized Valence Bond (GVB) model. Unlike the GVB, individual electron pairs in SSG may have many orbitals. This feature helps to describe effects of low-lying empty d-orbitals in transition metals. The SSG model is unique to Q-Chem.
The SSG model is defined both in spin-restricted and spin-unrestricted forms. The spin-unrestricted form is size-consistent, therefore is useful when one has to study the potential energy surface, or to compare the energy of molecule to that of its fragments. The individual electron pairs in the SSG model are usually localized where chemical intuition places chemical bonds, lone pairs, and core orbitals. Application of perturbation theory to the SSG reference state yields interactions between chemical bonds, such as dispersion and charge transfer.
Applications of the SSG model are limited by timing. The individual SCF iteration in SSG is 3 to 5 times slower than in DFT or Hartree-Fock. In addition, it is harder to converge the SSG wavefunction. Therefore, more iterations are needed in the SSG. The number of iteration can be as low as 50 or as high as a few thousands.
The SSG model recovers relatively modest amount of correlation energy. It does not include dispersion interactions and charge transfer effects. These are accounted for by applying perturbation theory to the SSG wavefunction.
The SSG model and its perturbation corrections are available in Q-Chem for all standard and user-defined basis sets.
Useful publications:
“A geminal model chemistry”, V. A. Rassolov, J. Chem. Phys.117(13) 5978 (2002)
“Geminal model chemistry II. Perturbative corrections”, V. A. Rassolov, F. Xu, and S. Garashchuk, J. Chem. Phys.120(22) 10385 (2004)