Small Al clusters. I. The effect of basis set and correlation on the geometry of small Al clusters

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Abstract
The effect of basis set and correlation on the bond length and atomization energy of Al2 and Al4 are studied. For Al13 the effect of basis set improvements is investigated only at the SCF level. A large valence basis set, such as a triple-zeta contraction of the Huzinaga (12s9p) primitive set, is needed to avoid significant superposition error and a poor description of the bonding. An ECP is developed from the Huzinaga basis, which accurately reproduces the all-electron results. It is also shown that an ECP developed from a (10s6p) primitive set reproduces the all-electron results in the same basis, but is in poor agreement with the ECP and all-electron results based upon the larger (12s9p) primitive set; this has implications on how ECP basis sets should be developed for the second row atoms. The d functions are found to be important for all clusters considered. At the SCF level, the addition of d polarization functions results in a sizable (about 0.2 a0) contraction of the Al–Al bond length and increases the dissociation energy per atom by about 0.25 eV. The inclusion of electron correlation results in a small bond contraction, but a large increase in the De per atom. The increase in De and reduction in re with the inclusion of electron correlation is almost independent of the number of d functions.