CCL: Isolated molecules & unoccupied orbitals: does the basis set matter?

[Ulf Ekström <uekstrom===gmail.com>]

 Sent to CCL by: =?ISO-8859-1?Q?Ulf_Ekström?= [uekstrom|-|gmail.com]
 > Is it possible that plane-wave and local-basis set codes behave
 > somehow differently concerning unoccupied orbitals? If so, why?
 Yes, because these (~Rydberg) orbitals can be very very large, and are
 typically not included in standard basis sets. In this sense the plane
 wave basis is better, until you hit the boundary of your unit cell. You
 don't mention which basis set you use, but if you try to increase it by
 adding more diffuse functions (some programs can do this auto-
 matically) you will most likely find your plane wave states also in your
 localized orbital calculations. It can happen that your state that is too
 high with the small basis will come down in energy with a more
 appropriate basis.
 Sincerely,
 Ulf Ekström, VU University Amsterdam.