26.4.5 Energy partitioning for molecular cluster calculations ENEPART

The local character of occupied and virtual orbitals in the local correlation treatment also offers the appealing possibility to decompose the intermolecular interaction energy of molecular clusters into individual contributions of different excitation classes. This allows to distinguish between intramolecular-, dispersive-, and ionic components of the correlation contribution to the interaction energy (cf. M. Schütz, G. Rauhut and H.J. Werner, J. Phys. Chem. 102, 5197 (1998)). The energy partitioning algorithm is activated either by supplying the global ENEPART card:

ENEPART,[epart],[iepart]

The epart parameter determines the cutoff distance for (intramolecular) bond lengths (in a.u., default 3 a.u.) and is used to automatically determine the individual monomer subunits of the cluster. The iepart parameter enables the energy partitioning, if set to a value larger than zero (default 1). Additionally, if iepart is set to 2, a list of all intermolecular pair energies and their components is printed.

The output section produced by the energy partitioning algorithm will look similar to the following example:

 energy partitioning enabled !
 centre groups formed for cutoff [au] = 3.00
  1   :O1  H11 H12
  2   :O2  H21 H22
 energy partitioning relative to centre groups:
 intramolecular correlation:      -.43752663
 exchange dispersion       :       .00000037
 dispersion energy         :      -.00022425
 ionic contributions       :      -.00007637

The centre groups correspond to the individual monomers determined for epart=3. In the present example, two water monomers were found. The correlation energy is partitioned into the four components shown above. The exchange dispersion, dispersion and ionic components reflect directly the related intermolecular components of the complex, while the intramolecular correlation contribution to the interaction energy has to be determined by a supermolecular calculation, i.e. by subtracting the (two) corresponding monomer correlation energies from the intramolecular correlation component of the complex given in the output.