# Basis Set Superposition Error along the Free-Energy Surface of the Water Dimer

journal contribution

posted on 2007-07-10, 00:00 authored by Jens Thar, Rainer Hovorka, Barbara KirchnerIn this article we review the behavior of static plane wave basis set calculations in
comparison to Gaussian basis set calculations. This was done in the framework of density
functional theory for description of hydrogen bonds with the water dimer as an example.
Furthermore we carried out molecular dynamics simulations enforcing the self-dissociation
reaction of the water dimer to study the influence of the basis set onto the reaction. Not
surprisingly, we find strongly varying results of the calculated forces for a chosen cutoff along
the reaction coordinates. The basis set superposition errors of the dimer interaction energy are
analyzed along the free-energy surface, i.e., along the trajectories. Based on the analysis along
the trajectories a qualitative and quantitative estimate depending on the particular point of the
free-energy surface can be provided. Namely, at the intermolecular O···H distance close to the
equilibrium geometry the errors are smaller than at shorter O···H distances. However, the
distribution at the equilibrium distance is more unsymmetrical than the distribution at short
distances. It is wider, and the standard deviation is larger than at shorter distances where the
basis set superposition error is larger.