geovar: constrained optimization, Linear Transit and NEB parameters

The block key GeoVar is used

• To put restrictions on the number of coordinates that are varied and
• To define Linear Transit or NEB parameters and assign them initial and final
  (and in case of NEB - also intermediate) values.

geovar can also be used to assign (initial) values to coordinates without other implications, but this feature is accidental.

In the input section of atomic coordinates (key ATOMS) identifiers (names) may be used rather than numerical values wherever coordinate values are expected: x, y, z in case of Cartesian coordinate input; r, q, f in case of internal coordinates. All such identifiers must then be specified under geovar and assigned a value.

GEOVAR
 Name Data
 ...
end

Name

An identifier that can be used in place of a numerical value for one or more of the atomic coordinate values under atoms.

Data

Either of the following three formats:

1 A single value simply assigns the value to the corresponding atomic coordinate(s).

2 Two or more values (separated by a delimiter) imply that the corresponding atomic coordinate is a Linear Transit or a Nudged Elastic Band parameter. For Linear Transit, only two values are allowed in which case they specify initial and final values of the LT path, respectively. In case of a NEB calculation one can provide more than just initial and final values to get a better initial approximation of the reaction path. It is generally recommended (and in some cases necessary) to use more values. Intermediate images will be obtained by polynomial interpolation of degree N-1, where N is the number of values.

3 A single value followed by a letter F assigns the value to the corresponding atomic coordinates and specifies that these coordinates are frozen: they will not be optimized.

As regards the optimization of coordinates other than the frozen ones and the LT or NEB parameters, the meaning and effect of the input under geovar depends on the subkey optim in the geometry block:

If selected has been set, optimizations are carried out only for the coordinates that are referred to under geovar (and that are not Linear Transit parameters or Frozen). All coordinates that were input as simple numerical data under atoms are kept frozen then.

Alternatively, if selected has not been set (: all, the default) all atomic coordinates are optimized (except the Linear Transit parameters and the explicitly frozen coordinates). In that case, each assignments under geovar other than to freeze the coordinate or to define it as a Linear Transit parameter simply assigns an initial value to the pertaining coordinates. In this respect it is not different from typing the numerical value directly in the atoms block, except for the next aspect. Please note that whereas during a linear transit run the LT parameters are never optimized, the NEB parameters specified in the geovar section are always optimized.

The same identifier may be used for two or more coordinates in atoms. If they are varied (i.e. if they are not frozen) they will forcibly be kept equal throughout the optimization so that they constitute only one degree of freedom. Don't use the same geovar variable for coordinates that belong to atoms of different chemical types or to different types of coordinates (an angle and a bond length for instance). It is not sensible to do so and it will very probably lead to an error abort or to stupid results.

It is allowed to put as atomic coordinate under atoms minus a geovar variable name, i.e. the name preceded directly by a minus sign (without a blank in between!). The coordinate will then be kept equal, but with opposite sign, to coordinates that are defined by the same variable without the minus sign. The initial (and final, in case of a LT or NEB run) value for that coordinate is the negative of the geovar value.