MP2, Tamm-Dancoff, and RPA methods based on the generalized HF solution

A Hartree–Fock (HF) method dealing with general spin orbitals (GSO) of the form ϕαα+ϕββ is called a generalized HF (GHF) method. This method can provide a characteristic electronic state with noncollinear spin configuration. Therefore, it is interesting to apply the method to spin-frustrated systems. In this study, we have developed ab initio GSO-based second-order Møller–Plesset (MP2), Tamm–Dancoff approximation (TDA), and random-phase approximation (RPA) methods. Further these methods, including GHF, were applied to the H3 molecule and obtained ground states and excited states of the molecule are discussed. The GSO-based MP2 method gave a similar energy surface to one of GHF. The GSO-based TDA and RPA methods found that excitations between the two states correspond to two different GHF solutions.