In versions of OPTIM prior to 1999
projection was used to remove displacements corresponding to
overall translations and rotations where necessary.[77,78]
From April 1999 projection was replaced by eigenvalue shifting where the
eigenvalues corresponding to eigenvectors which conserve the energy
are moved to the top of the spectrum. This procedure seems to be simpler
and more efficient, especially in terms of memory. A new keyword *SHIFT*
was introduced, which enables the upward shift to be adjusted if required.
For systems involving periodic boundary conditions no shifting should really be
needed, but there is little overhead for doing so and the program is simpler.
For trapped ion clusters
rotation-only shifting is used. If keyword *ROT* is specified then shifting is performed
only for the eigenvalues corresponding to
rotations around the space-fixed -axis. Otherwise shifting is applied to the
eigenvalues corresponding to both translation and rotation.
The program can cope with linear molecules (including diatomics) so long as all the atoms
are not in the plane. However, linear molecules and *ROT* will not work.

David Wales
2018-06-23