Subject: Energy raising by means of a basis enlargement at B3LYP/6-31G*

From: Luis Salvatella <lsalvate:at:posta.unizar.es>
Date: Thu, 04 Sep 2003 13:30:39 +0200

Dear CCLers,
 I have found a curious (?) result by trying to calculate the
counterpoise correction of a chemical system.
 Thus, the B3LYP/6-31G* calculation of a distorted cyclobutenedione
molecule yields an energy value of
-303.994148778 Hartrees.
 Z-matrix:
             C                    -1.246649   -0.481002    0.699675
             C                     0.030221   -1.258355    0.792789
             C                     0.030221   -1.258355   -0.792789
             C                    -1.246649   -0.481002   -0.699675
             O                     0.715293   -1.723751    1.667320
             O                     0.715293   -1.723751   -1.667320
             H                    -2.016598   -0.274511    1.431060
             H                    -2.016598   -0.274511   -1.431060

 If a ghost carbon atom is added, an energy decrease is observed
(-303.996434803 Hartrees).
 Cartesian coordinates:
             C                    -0.430472    1.721641    0.793937

 However, if a second ghost atom (either carbon or hydrogen) is
included, a significant energy raise is found
(-303.993557388 Hartrees and -303.993425450 Hartrees, respectively).
 Cartesian coordinates:
             C                    -0.430472    1.721641   -0.793937

 Can somebody explain this phenomenon? I know that B3LYP is a
non-variational method, but an energy raise of ca. 2
Kcal/mol is rather surprising.

 Thank you very much,

 Luis Salvatella
 University of Zaragoza
 Spain