9.3.1 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, $0\le value\le 180$; atom2 is the middle atom of the bend:
bend   atom1   atom2   atom3   value

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

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

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