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By performing numerically precise calculations on the He 2 , Ne 2 , Ar 2 , Kr 2 , HeNe, HeAr, HeKr, NeAr, NeKr, and ArKr diatomic molecules we have determined the capacity of three popular approximations to densityfunctional theory to accurately describe bonding in these rare-gas systems. The local-density approximation, the Perdew-Wang 1991 generalized-gradient approximation, and the Perdew-Burke-Ernzerhof generalizedgradient approximation are utilized in the calculation of equilibrium bond lengths, atomization energies, and anharmonic and harmonic vibrational frequencies. We also use the density-functional-based determination of atomic polarizabilities and ionization potentials to obtain the coefficients for the long-range (1/r 6 ) attraction. Our calculations suggest that the interaction from the overlap of atomic densities is the primary binding mechanism in these systems at short range but that the long-range 1/r 6 attraction could also contribute to the total binding energy.

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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 intensities͒ 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. ͓S0163-1829͑98͒05228-X͔