# 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 Ångstroms; value $>0$:
stre   atom1   atom2   value

angles
Values in degrees, $0\leq value\leq 180$; atom2 is the middle atom of the bend:
bend   atom1   atom2   atom3   value

out–of–plane–bends
Values in degrees, $-180\leq value\leq 180$ atom2; angle between atom4 and the atom1–atom2–atom3 plane:
outp   atom1   atom2   atom3   atom4   value

dihedral angles
Values in degrees, $-180\leq value\leq 180$; angle the plane atom1–atom2–atom3 makes with the plane atom2–atom3–atom4:
tors   atom1   atom2   atom3   atom4   value

coplanar bends
Values in degrees, $-180\leq value\leq 180$; bending of atom1–atom2–atom3 in the plane atom2–atom3–atom4:
linc   atom1   atom2   atom3   atom4   value

perpendicular bends
Values in degrees, $-180\leq value\leq 180$; bending of atom1–atom2–atom3 perpendicular to the plane atom2–atom3–atom4:
linp   atom1   atom2   atom3   atom4   value

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 $X$, $Y$ and $Z$ to specify the coordinate(s) to be fixed: $X$, $Y$, $Z$, XY, XZ, YZ, XYZ. The fixing characters must be next to each other. e.g.,

FIXED
2 XY
ENDFIXED