Computational models that use single reference wavefunction describe molecules in terms of independent electrons interacting via mean Coulomb and exchange fields. It is natural to improve this description by using correlated electron pairs, or *geminals*, as building blocks for molecular wavefunctions. Requirements of computational efficiency and size consistency constrain geminals to have [297], with each geminal spanning its own subspace of molecular orbitals [298]. Geminal wavefunctions were introduced into computational chemistry by Hurley, Lennard-Jones, and Pople [299]. An excellent review of the history and properties of geminal wavefunctions is given by Surjan [300].

We implemented a size consistent model chemistry based on Singlet type Strongly orthogonal Geminals (SSG). In SSG, the number of molecular orbitals in each singlet electron pair is an adjustable parameter chosen to minimize total energy. Open shell orbitals remain uncorrelated. The SSG wavefunction is computed by setting SSG *$rem* variable to 1. Both spin-restricted (RSSG) and spin-unrestricted (USSG) versions are available, chosen by the UNRESTRICTED *$rem* variable.

The wavefunction has the form

(5.45) | |||||

with the coefficients , , and subspaces chosen to minimize the energy

(5.46) |

evaluated with the exact Hamiltonian . A constraint for all MO coefficients yields a spin-restricted version of SSG.

SSG model can use any orbital-based initial guess. It is often advantageous to compute Hartree-Fock orbitals and then read them as initial guess for SSG. The program distinguishes Hartree-Fock and SSG initial guess wavefunctions, and in former case makes preliminary assignment of individual orbital pairs into geminals. The verification of orbital assignments is performed every ten wavefunction optimization steps, and the orbital pair is reassigned if total energy is lowered.

The convergence algorithm consists of combination of three types of minimization steps. The direct minimization steps [301] seeks a minimum along the gradient direction, rescaled by the quantity analogous to the orbital energy differences in SCF theory [297]. If the orbitals are nearly degenerate or inverted, a perturbative re-optimization of single geminal is performed. Finally, new set of the coefficients and is formed from a linear combination of previous iterations, in a manner similar to DIIS algorithm [163, 164]. The size of iterative subspace is controlled by the DIIS_SUBSPACE_SIZE keyword.

After convergence is achieved, SSG reorders geminals based on geminal energy. The energy, along with geminal expansion coefficients, is printed for each geminal. Presence of any but the leading coefficient with large absolute value (value of 0.1 is often used for the definition of “large”) indicates the importance of electron correlation in the system. The Mulliken population analysis is also performed for each geminal, which enables easy assignment of geminals into such chemical objects as core electron pairs, chemical bonds, and lone electron pairs.

As an example, consider the sample calculation of ScH molecule with 6-31G basis set at the experimental bond distance of 1.776 . In its singlet ground state the molecule has 11 geminals. Nine of them form core electrons on Sc. Two remaining geminals are:

*Geminal 10 E = -1.342609*

0.99128 -0.12578 -0.03563 -0.01149 -0.01133 -0.00398*Geminal 11 E = -0.757086*

0.96142 -0.17446 -0.16872 -0.12414 -0.03187 -0.01227 -0.01204 -0.00435 -0.00416 -0.00098

Mulliken population analysis shows that geminal 10 is delocalized between Sc and H, indicating a bond. It is moderately correlated, with second expansion coefficient of a magnitude 0.126. The geminal of highest energy is localized on Sc. It represents electrons and describes their excitation into orbitals. Presence of three large expansion coefficients show that this effect cannot be described within GVB framework [302].

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 [303, 304] 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 wavefunction to be an eigenfunction of the reference Hamiltonian [305].

All perturbative contributions to the SSG(EN2) energy (second-order Epstein-Nesbet perturbation theory of SSG wavefunction) 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 wavefunction. Formally, the SSG(EN2) correction can be applied both to RSSG and USSG wavefunctions. 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 wavefunction rarely exceeding 0.1 in magnitude.

SSG

Controls the calculation of the SSG wavefunction.

TYPE:

INTEGER

DEFAULT:

0

OPTIONS:

0

Do not compute the SSG wavefunction

1

Do compute the SSG wavefunction

RECOMMENDATION:

See also the UNRESTRICTED and DIIS_SUBSPACE_SIZE

$remvariables.