Abstract
In the many-body perturbation theory and related methods equations involve sums over orbital indices. Reduction of the many-body equations to such a form is not trivial when spin eigenfunctions are used. A method to achieve such reduction, based on the algebraical properties of the unitary group generators, is described. As an example of application a formula for the second order MBPT with an arbitrary open shell reference state of the RHF type is derived.