ENPART

This is a very useful tool for analyzing the energy terms within a system. The total energy, in eV, obtained by the addition of the electronic and nuclear terms, is partitioned into mono- and bi-centric contributions (if LARGE is present), and these contributions in turn are divided into nuclear and one- and two-electron terms (if LARGE is present). 

Atoms can be grouped into sets, up to 9 sets are allowed.  The format is ENPART(n1, n2,n3...) where n1, n2, n3, etc., are the number of atoms in each set.  All atoms in each set must be together in the geometry file.  For example, to do an energy partition on the water molecule, use atoms O, H, H, O, H, H, and use ENPART(3,3).  In this, the first three atoms are the first water molecule.

Here is an example of energy partitioning for the HF dimer.  In order to work out the HF - HF interaction energy, keyword ENPART(2,2) was used.

          TOTAL ENERGY PARTITIONING

          ALL ENERGIES ARE IN ELECTRON VOLTS

            ONE-CENTER TERMS 
          E-E:  ELECTRON-ELECTRON REPULSION
          E-N:  ELECTRON-NUCLEAR ATTRACTION


 
 ATOM      E-E       E-N    (E-E + E-N)
   F   1   333.3949 -800.5397 -467.1449  
   H   2     1.8754   -8.1040   -6.2286
   F   3   333.3956 -800.5406 -467.1450
   H   4     1.8754   -8.1039   -6.2285