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  Role of basis functions and symmetry.
    I am not sure I understand the question, but here are some factors
 to consider. First the make up of virtual orbitals is determined by
 two requirements. They must be orthogonal to the occupied orbitals
 and to each other.If you add extra functions which have a different
 symmetry than the ones already present they could apear;above,inbetween
 or below the valence or antibonding manifold. If you add lots of very
 diffuse functions they will probably be above the antibonding mos . then
 they will depress those of the same symmetry and leave the others alone.
   If you have a molecule with low lying RYDBERG states these are
 usually described by adding suitable diffuse functions which will
 then be beloww the antibonding valence manifold. Then if they mix
 much then because of orthogonality they will push up the valence virtual
 orbitals of the same symmetry.
 The only way to really tell is to look at the LCAO make up of your
 virtual MO, particularly if you have a basis set which describes
 states, they could be anywhere! John E. BLOOR(PA13808 AT UTKVM1)