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
The one-center energy of each atom is in the column (E-E + E-N) Unless you're REALLY interested in it, the component parts, E-E and E-N, are not useful.
The two-center energies of each atom pair is in the column EE. Unless you're REALLY interested in it, the component parts are not useful.
Energy of the first HF = one-center energy of F 1 (-467.145) + one-center energy of H 2 (-6.229) + two-center energy of H 2 - F 1 interaction (-11.866)
Energy of the second HF = one-center energy of F 3 (-467.145) + one-center energy of H 4 (-6.229) + two-center energy of H 4 - F 3 interaction (-11.866)
Energy of interaction of the first HF with the second HF = two-center energies of F 3 - F 1 - H interaction (0.321) + F 3 - H 2 (-0.350) + H 4 - F 1 (-0.350) + H 4 - H 2 (0.341) = -0.039eV
TOTAL OF ONE-CENTER TERMS = ELECTRON-NUCLEAR (ONE-ELECTRON) + ELECTRON-ELECTRON (TWO-ELECTRON)
TOTAL ELECTROSTATIC INTERACTION = ELECTRON-ELECTRON REPULSION + ELECTRON-NUCLEAR ATTRACTION + NUCLEAR-NUCLEAR REPULSION
GRAND TOTAL OF TWO-CENTER TERMS = (EXCHANGE + RESONANCE ENERGY) + TOTAL ELECTROSTATIC INTERACTION
ETOT (EONE + ETWO) = TOTAL OF ONE-CENTER TERMS + GRAND TOTAL OF TWO-CENTER TERMS = TOTAL ENERGY just after FINAL HEAT OF FORMATION in output