Spin projected and improved energy states from unrestricted Hartree–Fock

A simple approach for improving the energy and quality of the unrestricted Hartree–Fock (UHF) approximation is presented. Starting from the UHF Slater determinant (SD), we extract eigenstates of S 2 by means of which the energies are calculated. Some of these energies are lower than that of UHF, thus giving a correlation correction. Because of the simplifications made, the present methodology is applicable for large molecules, where other methods are costly or unreliable. To test our procedure we made applications to small atoms and molecules, where it is easy to control errors and compare results.