For many-electron systems, among several possible symmetry-forcing schemes, only symmetrized Rayleigh-Schrödinger perturbation theory (SRS), was developed up to the thirdorder. SRS employs the weak symmetry forcing, which is achieved by inserting the antisymmetrizer into the energy expressions for the E (n) pol corrections of the polarisation theory. This procedure results in splitting the energy corrections of each order into two parts