Spurious jumps

Problem: during geometry optimization, the atomic configuration makes a large and unrealistic jump.

Possible cause 1: the triplet of atoms to which the current atom is related by the Z-matrix is (almost) co-linear. When, in a geometry step, the triplet passes through co-linearity, the dihedral angle for the current atom should make a discontinuous jump of 180 degrees. This is not checked in the program and the dihedral angle may not get corrected, resulting in a geometric jump of the atom (and hence of all atoms related to it by the Z-matrix).

Check: the triplets of atoms, used in your Z-matrix to define the dihedral angles. If one of them is almost colinear, then:

Cure: redefine the Z-matrix or use Cartesian optimization.

Alternative cause 2: the connectivity of the Z-matrix does not reflect the important bonds. Especially when the molecule contains (a large number of) rings, this badly affects the stability of the geometry update step. The reason is basically that computed Cartesian forces are transformed into changes of the curvilinear internal coordinates. The transformation between the two systems of coordinates is non-linear, but mathematically assumed to be linear. This is only a good approximation for small steps.

Cure: redefine the Z-matrix and/or (if the geometry steps are very large) set a smaller upper bound on the maximum step (key GEOMETRY, subkey step).