40.2.2 Optimization coordinates (COORD)

It is possible to use various coordinate types and algorithms for the optimization. This can be controlled by additional subcommands as described in this and the following subsections.

COORD,[opt_space],[opt_coord],[NOROT]

These options choose the optimization space and the coordinate system in which the optimization takes place.

opt_space defines the parameters to be optimized. By default, if the geometry input is given in Z-matrix format, all variables on which the Z-matrix depends are optimized. Subsets of the variables on which the Z-matrix depends can be chosen using the ACTIVE or INACTIVE subdirectives. If the Z-matrix depends on no variables or xyz input is used, all $3N$ cartesian coordinates are optimized.

opt_space can be one of the following:

ZMAT

Optimize all variables on which the Z-matrix depends (default if the geometry is given as Z-matrix).

3N

Optimize all $3N$ cartesian coordinates (default if the Z-matrix depends on no variables, or if xyz-input is used). Z-Matrix input coordinates will be destroyed if 3N is used..

opt_coord determines the coordinates in which the optimization takes place. By default, local normal coordinates are used. Optionally cartesian coordinates or natural internal coordinates can be used.

opt_coord can be one of the following:

NORMAL

Optimization in local normal coordinates. This is default if the Model Hessian is used to approximate the Hessian.

NONORM

Don't use local normal coordinates.

BMAT[=filename]

Use Pulay's natural internal coordinates, see G. Fogarasi, X. Zhou, P. W. Taylor and P. Pulay J. Am. Chem. Soc. 114, 8191 (1992); P. Pulay, G. Fogarasi, F. Pang, J. E. Boggs J. Am. Chem. Soc. 101, 2550 (1979)). Optionally, the created coordinates as well as additional informations about this optimization are written to the specified file. These coordinates resemble in part the valence coordinates used by vibrational spectroscopist, and have the advantage of decreasing coupling between different modes. This often increases the speed of convergence. The use of this option is highly recommended, especially in minimization of large organic molecules with rings. Nevertheless you should keep in mind that these coordinates are constructed automatically, and there exist exotic bond structures which might not be treated properly (e.g. weakly bonded species as in transition state optimizations). In such a case, if the BMAT optimization converges slowly or leads to symmetry-breaking errors, you should try another optimization method and/or cartesian or Z-Matrix coordinates.

If the option [NOROT] is given, the cartesian coordinates are not transformed to minimize rotations.