Eigenvalue Shifting

In versions of OPTIM prior to 1999 projection was used to remove displacements corresponding to overall translations and rotations where necessary.[43,44] 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 parameter 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 $z$-axis. Otherwise shifting is applied to the eigenvalues corresponding to both $z$ translation and rotation.

The program can cope with linear molecules (including diatomics) so long as all the atoms are not in the $z=0$ plane. However, linear molecules and ROT will not work!