C.2 Geometry Optimization with General Constraints
CONSTRAINT and ENDCONSTRAINT define the beginning and end, respectively, of the constraint section of $opt within which users may specify up to six different types of constraints:
interatomic distances
Values in angstroms; value 
:
stre atom1 atom2 value
angles
Values in degrees, 
; atom2 is the middle atom of the bend:
bend atom1 atom2 atom3 value
out–of–plane–bends
Values in degrees, 
atom2; angle between atom4 and the atom1–atom2–atom3 plane:
outp atom1 atom2 atom3 atom4 value
dihedral angles
Values in degrees, 
; angle the plane atom1–atom2–atom3 makes with the plane atom2–atom3–atom4:
tors atom1 atom2 atom3 atom4 value
coplanar bends
Values in degrees, ; bending of atom1–atom2–atom3 in the plane atom2–atom3–atom4:
linc atom1 atom2 atom3 atom4 value
perpendicular bends
Values in degrees, ; bending of atom1–atom2–atom3 perpendicular to the plane atom2–atom3–atom4:
linp atom1 atom2 atom3 atom4 value
C.2.1 Frozen Atoms
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:
atom coordinate_reference
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