Moller-Plesset perturbation theory derivatives, electronic energy

We state without further derivation that the electronic energy corrected to second order in Moller-Plesset perturbation theory, mp2 is... 

Using a single determinant to derive the Hartree-Fock wavefunction does not take into account the correlation between electrons of different spins. If the wavefunction is described by a linear combination of determinants, then configuration interaction is incorporated. Another method to model correlation energy involves Moller-Plesset perturbation theory. A more detailed description of this may be found in Ref. 41. 

A few quantum-chemical ab initio calculations dealt with the electronic structure and some molecular parameters of PH". Accordingly, the ground state, derived from that of the neutral molecule by adding a 2n (P3p7r) electron, is KL (4a)2 (5o) (2n), X rij [7]. The best theoretical value so far for the total molecular energy at the equilibrium internuclear distance, Et = -341.46363 Eh, was obtained from a recent G2 calculation by using a method that treats the electron correlation by Moller-Plesset perturbation theory (MP4) and quadratic configuration interaction (QCI) [8]. 

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