A Hartree-Fock (HF) method dealing with general spin orbitals (GSO) of the form phi (alpha)alpha + phi (beta)beta 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 Moller-Plesset (MP2), Tamm-Dancoff approximation (TDA), and random-phase approximation (RPA) methods. Further these methods, including GHF, were applied to the H-3 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. (C) 2000 John Wiley & Sons, Inc.