VIBRATIONAL FREQUENCIES AND INTENSITIES OF SMALL MOLECULES - ALL-ELECTRON, PSEUDOPOTENTIAL, AND MIXED-POTENTIAL METHODOLOGIES

We describe a numerically robust strategy for implementing pseudopoten tials in a way that is independent of the type of basis set employed. Applications of this method are illustrated for many small molecules a nd compared to all-electron calculations determined from the same comp uter code. In addition we introduce a mixed methodology that allows fo r the treatment of some atoms within an all-electron methodology and s ome atoms within a pseudopotential methodology. The type of bonding st udied here includes both ionic and covalent systems and includes atoms from each of the first four rows of the periodic table. In addition t o binding energies, structural parameters, electronic structures, and vibrational frequencies we compare vibrational intensities (e.g., Rama n and infrared intensities) obtained from LDA within all-electron, mix ed, and pseudopotential methodologies. We discuss issues related to ti ming for pseudopotentials versus all-electron when used in conjunction with local-orbital-based methods.