Spin projected extended Hartree--Fock calculations by direct minimization of the energy functional

A practicable SEHF scheme for the treatment of electron correlation in systems for which large scale configuration interaction calculations are prohibitive is presented. A study is made of the electronic structures to which the SEHF method may be most profitably applied. In order to illustrate this study and to provide a numerical demonstration of the minimization techniques ab initio SEHF calculations on a number of ten electron systems are reported. These calculations were performed for the singlet ground states of Ne, HF, H2O, NH3, CH4, and cyclic H10