Regularized Orbital-Optimized Møller-Plesset Perturbation Theory for Strongly Correlated Species
- Unrestricted Hartree-Fock (UHF); may exhibit artificial spin-symmetry-breaking in open-shell systems, and using UHF orbitals for correlated wave function calculations could lead to incorrect energetics and properties.
- Orbital-optimized MP2 (OOMP2) restores spin-symmetry and remove the discontinuity in the first-order properties at the onset of symmetry-breaking in MP2.
- Orbital optimization in OOMP2 could produce very small energy denominators which make the total energy diverge. In addition, OOMP2 may unphysically prefer restricted solutions and remove the Coulson-Fischer point at potential energy curves.
- Q-Chem offers several regularization schemes (κ-OOMP2 and σ-OOMP2) to fix the issues above. Combined with approximate spin-projection, these regularized OOMP2 methods are powerful as well as economical tools in studying strongly correlated systems.
Measures of symmetry breaking parameters as functions of the regularization strength of C30