Zero of energy used in the Multi Electron Configuration Interaction

The energy of the system after all the electronic terms arising from the electrons of the M.O.s involved in the starting configuration are removed is a useful quantity. Removal of these terms lowers the orbital energies thus:

\begin{displaymath}\epsilon_{ii}^+ = \epsilon_{ii} -\sum_{j=B}^A(J_{ij}-K_{ij})O_j^g.

The arbitrary zero of energy in a MECI calculation is the starting ground state, without any correction for errors introduced by the use of fractional occupancies. In order to calculate the energy of the various configurations, the energy of the vacuum state (i.e., the state resulting from removal of the electrons used in the C.I.) needs to be evaluated. This energy is given by:

\begin{displaymath}GSE=E_g^+ = - \sum_{i=B}^A2\epsilon_{ii}^+O_i^g+J_{ii}(O_i^g)^2+

(Within the MECI routine, GSE refers to Eg+.)

By redefining the system so that those filled M.O.s which are not used in the MECI are considered part of an unpolarizable core, the new energy levels  εi- can be identified with the one-electron energies Hii and the total electronic energy Er of any microstate is set equal to the sum of the energy of the electrons considered in the microstate plus Eg+.