MOLPRO

42 GEOMETRY OPTIMIZATION (OPTG) 329
4
3
RCN
RNH
ALPHA
2
1
0
-3 -2 -1 0 1 2
IRC
42.4.7 Optimizing counterpoise corrected energies
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 OPTG 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.