On the importance of the biquadratic terms in exchange coupled systems: A post-HF investigation

The importance of the many-body interactions was studied both at the experimental and theoretical level. However, no definitive proofs of their presence as high order terms in the Heisenberg–Dirac–Van Vleck (HDVV) Hamiltonian, as far as we know, were presented. The present work wants to reanalyze the HDVV Hamiltonian at the Full CI level on the two model systems N2He and N3He3.