ON THE ACCURACY OF DENSITY FUNCTIONALS AND THEIR BASIS-SET DEPENDENCE - AN EXTENSIVE STUDY ON THE MAIN-GROUP HOMONUCLEAR DIATOMIC-MOLECULES LI-2 TO BR-2

The equilibrium bond distances, harmonic frequencies, and bond dissociation energies of the 21 homonuclear diatomics Li-2-F-2, Na-2-Cl-2, and K-2-Br-2 have been determined using approximate density functional theory (DFT) employing various widely used functionals and basis sets ranging from single zeta to triple zeta plus polarization quality. The results are in general much less sensitive to the size of the basis set as in conventional nb initio molecular orbital (MO) theory, while the choice of the functional is of much more significance. For one basis set (6-311G*), the performance of the DFT-based calculations has been compared and found to be superior to Hartree-Fock (HF) Moller Plesset second order perturbation theory (MP2), or configuration interaction with single and double excitations (CISD) calculations. Particularly, no pathological cases, such as the group 2 dimers (Be-2, Mg-2, Ca-2), are observed. (C) 1995 by John Wiley and Sons, Inc.