Quadratic configuration interaction with structure

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]. 

An extensive quantum-mechanical study that included correlation by second-order Meller-Plesset perturbation and quadratic configuration-interaction methods showed that the cluster species (NH4)2 is bound with respect to 2 NH4 by 7.5 to 9.7 kcal/mol, but unstable with respect to 2 NH3 + H2 by 86 to 89 kcal/mol. The bonding results from the interaction of the 3sai Rydberg orbitals of NH4. A minimum-energy structure of C2h symmetry was predicted. 

To illustrate the gauge invariant reference section for MAB, let us revisit the linear + quadratic E< e Jahn-Teller effect, which is known to exhibit a nontrivial MAB structure. There, the symmetry induced degeneracy of two electronic states (E) is lifted by their interaction with a doubly degenerate vibrational mode (e). In the vicinity of the degeneracy point at the symmetric nuclear configuration, this may be modeled by the vibronic Hamiltonian [39]... 

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