
Physics 2012
Projected HartreeFock TheoryDOI: 10.1063/1.4705280 Abstract: Projected HartreeFock theory (PHF) has a long history in quantum chemistry. PHF is here understood as the variational determination of an Nelectron broken symmetry Slater determinant that minimizes the energy of a projected state with the correct quantum numbers. The method was actively pursued for several decades but seems to have been abandoned. We here derive and implement a "variation after projection" PHF theory using techniques different from those previously employed in quantum chemistry. Our PHF methodology has modest meanfield computational cost, yields relatively simple expressions, can be applied to both collinear and noncollinear spin cases, and can be used in conjunction with deliberate symmetry breaking and restoration of other molecular symmetries like complex conjugation and point group. We present several benchmark applications to dissociation curves and singlettriplet energy splittings, showing that the resulting PHF wavefunctions are of high quality multireference character. We also provide numerical evidence that in the thermodynamic limit, the energy in PHF is not lower than that of brokensymmetry HF, a simple consequence of the lack of size consistency and extensivity of PHF.
