Collect. Czech. Chem. Commun. 1988, 53, 1953-1960

On the accuracy of conventional calculations of potential energy surfaces

Ulf Landscheid and Werner Kutzelnigg

Lehrstuhl für Theoretische Chemie, Ruhr-Universität Bochum, 4630 Bochum, F.R.G.


It is demonstrated for the example of H2+ that with the conventional basis expansion method (in terms of Gaussians) with fixed non-linear parameters the error of a computed potential curve varies strongly with the geometry. If the non-linear parameters are optimized for the equilibrium geometry, much larger errors arise for other geometries. This is very pronounced if only one polarization function of a given l is added to the basis, and qualitatively the same problem arises for any new l. The dependence of the error on the distance may also be oscillatory. For the example of Li2 in the SCF or an MC-SCF approximation essentially the same effects are observed. Here in addition substantial errors for large R may arise in the SCF curve if the basis is not able to describe the spurious ionic contributions.