THREE ATOMS BEING USED TO DEFINE...

If the internal coordinates of an atom depend on the dihedral angle it makes with three other atoms, and those three atoms fall in an almost straight line, then a small change in the Cartesian coordinates of one of those three atoms can cause a large change in its position. This is a potential source of trouble, and the data should be rearranged to make the geometric specification of the atom in question less ambiguous.

This message can appear at any time, particularly in reaction path and CHN calculations.

An exception to this rule is the case where three atoms fall exactly on a straight line. For example, if in propyne, the hydrogens are defined in terms of the three carbon atoms, then no error will be flagged. In such a system the three atoms in the straight line must not have the angle between them optimized, as the finite step in the derivative calculation would displace one atom off the straight line and the error-trap would take effect.

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