1. Interatomic distances must be greater than zero. Zero Ångstroms is acceptable only if the parameter is symmetry-related to another atom, and is the dependent function.

2. Angles must be in the range 0.0 to 180.0, inclusive. This constraint is for the benefit of the user only; negative angles are the result of errors in the construction of the geometry, and angles greater than 180 degrees can easily give rise to errors in the dihedral angles.

3.
Dihedral angles must be definable. If atom *i* makes a dihedral with
atoms *j*, *k*, and *l*, and the three atoms *j*, *k*, and *l* are in a
straight line, then the dihedral has no definable angle. During the
calculation this constraint is checked continuously, and if atoms *j*, *k*,
and *l* lie within 0.02 Ångstroms of a straight line, an attempt will
be made to re-number the connectivity. If this fails, the calculation will
output an error message and then stop. The exceptions to this constraint
are:

(a) if the angle is 0 or 180 degrees, in which case the dihedral is not used.

(b)
if atoms *j*, *k*, and *l* lie in an exactly straight line
(usually the result of a symmetry constraint), as in acetylene,
acetonitrile, but-2-yne, etc.

If the exceptions are used, care must be taken to ensure that the
program does not violate these constraints during any optimizations or
during any calculations of derivatives--see also `FORCE`.