Absolute atom positions can be frozen with the FIXED section. The section starts with the FIXED keyword as the first line and ends with the ENDFIXED keyword on the last. The format to fix a coordinate or coordinates of an atom is:
coordinate_reference can be any combination of up to three characters , and to specify the coordinate(s) to be fixed: , , , XY, XZ, YZ, XYZ. The fixing characters must be next to each other. e.g.,
FIXED 2 XY ENDFIXED
means the -coordinate and -coordinate of atom 2 are fixed, whereas
FIXED 2 X Y ENDFIXED
will yield erroneous results.
Note: When the FIXED section is specified within $opt, the optimization will proceed in Cartesian coordinates.