Epstein-Nesbet zero-order Hamiltonian

Arrays in which the calculated energy components will be stored are set to zero, etotal will contain the total energy whilst etwo will contain the two-body component, iz = 1 corresponds to the Hartree-Fock model zero order hamiltonian, that is the Moller-Plesset expansion whereas iz = 2 identifies the shifted denominator scheme which uses the Epstein-Nesbet zero order hamiltonian. ediag will be used to store the diagonal components. These energies are stored in the common block ptres together with the orbital energy (eorb (60)). 

The third order ring energy component for the perturbation series corresponding to the Epstein-Nesbet zero-order hamiltonian is given by... 

For single reference perturbation theory, there is a choice of reference hamil-the Moller-Plesset and Epstein-Nesbet zero-order hamiltonians were two choices considered in the early literature (see, for example, Ref. 54). 

A related perturbation scheme is based on the Epstein-Nesbet partitioning of the Hamiltonian, where the zero-order operator contains those parts of the Hamiltonian that conserve the spin-orbital occupations... 

As for the Fock operator, the eigenstates of the zero-order Epstein-Nesbet Hamiltonian are determinants. Although occasionally used, the Epstein-Nesbet approach has been considerably less successful than that of MPPT. 

In addition to our earlier work (9,54), we wish to point out that we can also calculate the first-order coefficients by relying entirely on perturbation theory. Of course, the result will very much depend on the way we partition the Hamiltonian H into the unperturbed part Hq and the perturbation W, H -Hq+W. Since our zero-order wave function is assumed to represent a general multi-configurational Cl wave function, it is easier to employ the Epstein-Nesbet (EN) type perturbation theory. For this purpose we choose the unperturbed Hamiltonian Hq as follow... 

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