Geometry optimization of counterpoise corrected energies is possible by performing for the total system as well as for each individual fragment separate FORCE calculations. The gradients and energies are added using the ADD directive. This requires that NOORIENT has been specified in the geometry input, in order to avoid errors due to unintended rotation of the system. This default can be disabled using the NOCHECK option, see ADD above.
The way a counterpoise corrected geometry optimization works is shown in the following example. Note that the total counterpoise corrected energy must be optimized, not just the interaction energy, since the interaction energy depends on the monomer geometries and has a different minimum than the total energy. The interaction energy could be optimized, however, if the monomer geometries were frozen. In any case, the last calculation before calling OPT must be the calculation of the total system at the current geometry (in the example below the dimer calculation), since otherwise the optimizer gets confused.
Input: hfdimer_cpcopt1.com Output: hfdimer_cpcopt1.out
In the next example the monomer structures are kept fixed, and the interaction energy is optimized.
Input: hfdimer_cpcopt2.com Output: hfdimer_cpcopt2.out
P.J. Knowles and H.-J. Werner