Subject: BITNET mail follows

From: <[email-masked]>
Date: 11 Apr 95 11:06 LCL

A number of researchers have reported problems with ab initio
  calculations such as unrealistic charges, large changes on changing the
  method( Basis set or inclusion of correlation by diff methods, lack of
  convergence etc). It is POSSIBLE that these problems all have a
  common source, namely the existence of low lying virtual orbitals.
  This canlead to a different order in the eigenvalues from one iteration
  to another   and since most programs use the aufbau principle to
  fill up the MOs the type of MOS occupied can change during the
  calculation.This can be common in systems containing metal atoms
  wwith unfilled D orbitals or when 3D and 4S have similar energies. In
  the case of molecules with lowlying unoccupied MOs such as nitro
  compounds a small basis set can give an incorrect ground state
  at the HF level and then of course will also give nonsense on going to
   MP2 level. One needs to look at the occupied MOs from iteration to
   iteration to see if a) the correct MOs are occupied and b) whether
   there is a switch between occ and unocc MOs between diff calculations.
    If there is you have to remedy the situation by freezing the occupati
     ion numbers or use some shift parameter to separate the energies
   of the occupied MOs from the unocupied ones. This used to be
   explained in earlier Gaussian manuals eg G90 so I assume there
   is something on it in the later versions.
                                John E. Bloor(UTK TENN)