Geometry parameters

The ANGSTROM directive

The nuclear coordinates and distance parameters used to define the geometry are assumed by default to be in atomic units. This can be over-ridden using the ANGSTROM directive. The conversion factor is 1 bohr = 0.529177249 angstrom.

VARIABLES and CONSTANTS

Parameters used to specify the geometry can be given in terms of VARIABLES or CONSTANTS. These differ only in their effect in any subsequent geometry optimization; the values of variables are optimized whereas those of constants remain fixed. It is also possible to give values explicitly in the ATOMS directive; such values also remain constant. The only exception to this is that if no variables are declared at all (i.e all the the geometry information is given explicitly within the ATOMS directive) and an optimization is called for, then all geometrical parameters are varied in the optimization.

If no unit is provided the values are assumed to represent distances in either bohr (default) or ångstrom if the ANGSTROM directive has been used. Individual parameters may be given units: A for Angstrom, B for bohr, D for degrees or R for radians. Angles should always have a unit.

```     VARIABLES
name  value  <unit>
name  value  <unit>
.
.
END

CONSTANTS
name  value  <unit>
name  value  <unit>
.
.
END```
The use of variables and constants will be much clearer if you look at a few example datasets, either those given in this chapter, or those in the chapter on geometry optimisations.

Dihedral angle convention

DIHEDRAL OLD / NEW

There are two main conventions for defining the signs of dihedral angles. This keyword has been included to allow the possibility of a change of definition in the future. At present the definition in CADPAC6 is the same as that in CADPAC5. Thus the default is 'OLD'. This may be changed in future editions. For a description of the way dihedral angles are defined see the definitions of internal coordinates given later in this chapter.
Note that the dihedral angles are not printed by default. They can be printed using the command
PRINT DIHEDRAL.

The BASIS keyword and generic basis sets

BASIS basisname

for example,

BASIS 631G*

or,

BASIS DZP

This keyword indicates that a standard basis set is to be used in the calculation. The restrictions are that the basis must be one of the very standard generic types, that the same level of basis set must be used for every atom in the system, that extra basis functions cannot be added to modify the library definitions, and that one cannot use this keyword when doing basis set superposition corrections via the ghost atom technique (because the program needs to know the true atomic number in order to find the right basis set). A more flexible, but more complicated, basis set specification without these restrictions is available if the LIBRARY directive is used atom-by-atom. A full list of the basis sets available is given later when the basis set library is described.