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
   
   
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.
TWO-CENTER TERMS J: RESONANCE ENERGY E-E: ELECTRON-ELECTRON REPULSION K: EXCHANGE ENERGY E-N: ELECTRON-NUCLEAR ATTRACTION N-N: NUCLEAR-NUCLEAR REPULSION C: COULOMBIC INTERACTION = E-E + E-N + N-N EE: TOTAL OF ELECTRONIC AND NUCLEAR ENERGIES ATOM J K E-E E-N N-N C EE PAIR H 2 F 1 -12.1007 -4.8374 51.3351 -121.4430 75.1795 5.0717 -11.8664 F 3 F 1 0.0000 -0.0002 223.2326 -429.5659 206.6544 0.3210 0.3208 F 3 H 2 -0.0012 -0.0001 23.5576 -55.3454 31.4388 -0.3490 -0.3503 H 4 F 1 -0.0012 -0.0001 23.5496 -55.3269 31.4284 -0.3489 -0.3502 H 4 H 2 -0.0015 -0.0001 2.2758 -6.3168 4.3835 0.3425 0.3409 H 4 F 3 -12.0999 -4.8372 51.3339 -121.4406 75.1776 5.0709 -11.8662 System is to be split into 2 parts Part No. of atoms Atoms 1 2 1 to 2 2 2 3 to 4 Part 1 self - energy (ONE-CENTER(E-E + E-N) + EE): -485.2398 Part 2 self - energy (ONE-CENTER(E-E + E-N) + EE): -485.2398 Part 2 - part 1 interaction energy (EE): -0.0388 Note on interaction energies
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
*** SUMMARY OF ENERGY PARTITION *** --------------------------------------- ONE-CENTER TERMS ELECTRON-NUCLEAR (ONE-ELECTRON) -1617.2882 EV ELECTRON-ELECTRON (TWO-ELECTRON) 670.5412 EV TOTAL OF ONE-CENTER TERMS -946.7470 EV --------------------------------------- TWO-CENTER TERMS RESONANCE ENERGY -24.2044 EV EXCHANGE ENERGY -9.6751 EV EXCHANGE + RESONANCE ENERGY: -33.8795 EV ELECTRON-ELECTRON REPULSION 375.2846 EV ELECTRON-NUCLEAR ATTRACTION -789.4387 EV NUCLEAR-NUCLEAR REPULSION 424.2622 EV TOTAL ELECTROSTATIC INTERACTION 10.1081 EV GRAND TOTAL OF TWO-CENTER TERMS -23.7714 EV --------------------------------------- ETOT (EONE + ETWO) -970.5184 EV