In an attempt to obtain an SCF by damping oscillations which slow
down the convergence or prevent an SCF being achieved, the virtual M.O.
energy levels are shifted up or down in energy by a shift technique [49].
The
principle is that, if the virtual M.O.s are changed in energy relative to
the occupied set, then the polarizability of the occupied M.O.s will
change pro rata. Normally, oscillations are due to autoregenerative
charge fluctuations.
The SHIFT method has been re-written so that the value of SHIFT
changes automatically in an attempt to optimize convergence. This can
result in a positive or zero shift of the virtual M.O. energy levels.