Vibrational frequencies and intensities of small molecules: All-electron, pseudopotential, and mixed-potential methodologies

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