THE PERFORMANCE OF A VARIETY OF DENSITY-FUNCTIONAL AND AB-INITIO METHODS ON THE STRUCTURAL PROBLEM OF BERYLLIUM BOROHYDRIDE

The D-2d and D-3d isomers of beryllium borohydride (BeB2H8) have been studied with different ab initio and density functional techniques. Op timized geometries were obtained with a polarized split valence basis set and electronic energies were calculated at the optimum geometry wi th an extended polarized basis set containing f functions. Both MP2 an d MP4 calculations with the extended basis set indicate the D-3d confo rmer to be slightly more stable at the extended basis set level. The p erformance of many combinations of local and non-local exchange and co rrelation density functionals have been tested on this difficult struc tural problem. Only the non-local functionals approached the accuracy of ab initio MP2 calculations.