Note

This website is presently under development. The primary website for MOPAC is still http://openmopac.net until the majority of its contents have been migrated here.

`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 <large.html>`__ is present), and these contributions in turn are divided into nuclear and one- and two-electron terms (if `LARGE <large.html>`__ 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

ENPART¶

`ENPART`¶