46 GEOMETRY OPTIMIZATION (OPTG)

Automatic geometry optimization is invoked using the `OPTG` command. The `OPT`
command available in previous MOLPRO versions is no longer needed and not available any
more.

`OPTG`[, *key1=value, key2=value,......*]

The `OPTG` command can be used to perform automatic geometry optimizations for all kinds
of wavefunctions. For minimum searches, it is usually sufficient to give just the `OPTG` command
without further options or directives, but many options are available which are described in
the following sections.

Various optimization methods can be selected as described in section 46.2.1.
MOLPRO allows minimization (i.e. search for equilibrium geometries),
transition state optimization (i.e. search for saddle points on energy surfaces),
and reaction path following.
The standard algorithms are based on the *rational function*
approach and the *geometry DIIS* approach.
Also available is the *quadratic steepest descent following* method of Sun and Ruedenberg
(see J. Sun and K. Ruedenberg, *J. Chem. Phys.* **99**, 5257 (1993)). This method is
often advantageous in Transition State searches.
For a detailed discussion of the various minimization algorithms see
F. Eckert, P. Pulay and H.-J. Werner, *J. Comp. Chem* **18**, 1473 (1997).
Reaction path following is described in F. Eckert and H.-J. Werner, *Theor. Chem. Acc.* **100**, 21, (1998).
Please refer to the references section for citations of the analytic gradient methods.

When analytical gradients are available for the optimized energy these will be used. See section 45.1 for
a list of methods with analytical gradients.
Otherwise the gradient will be computed numerically from finite energy differences.
Normally, the last computed ground-state energy is used. But the `VARIABLE`
directive or option can be used to optimize, e.g., Davidson corrected energies, excited states,
or counterpoise corrected energies.

By default the program repeats in each geometry optimization step those commands in the input that are needed to compute the last energy. For example, for MP2 gradients the commands HF and MP2 are needed. The MP2 gradients will then be computed automatically. It is also possible to define procedures for the energy calculation, or to specify the first command from which the input should be repeated in each step (see section 46.1.1). The section of the input which is needed for the geometry optimization must not modify variables that are used in the geometry definition (changes of such variables are ignored, and a warning message is printed).