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###

46.4.7 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.

hfdimer_cpcopt1.com

The next example shows how the same calculations can be done using numerical gradients. In this
case, first the total counter-poise corrected energy is formed and then optimized. Note that
the `ADD` command does not work for numerical gradients.

hfdimer_cpcopt1_num.com

In the last example the monomer structures are kept fixed, and the interaction
energy is optimized.

hfdimer_cpcopt2.com

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molpro@molpro.net 2018-08-14