Extension of Gaussian-2 (G2) theory to bromine- and iodine-containing molecules: Use of effective core potentials

Abstract

Basis sets have been developed for carrying out G2 calculations on bromine- and iodine-containing molecules using all-electron (AE) calculations and quasirelativistic energy-adjusted spin–orbit-averaged seven-valence–electron effective core potentials (ECPs). Our recommended procedure for calculating G2[ECP] energies for such systems involves the standard G2 steps introduced by Pople and co-workers, together with the following modifications: (i) second-order Mo/ller–Plesset (MP2) geometry optimizations use polarized split-valence [31,31,1] basis sets for bromine and iodine together with 6-31G(d) for first- and second-row atoms; (ii) single-point higher-level energies are calculated for these geometries using our new supplemented bromine and iodine valence basis sets along with supplemented 6-311G and McLean–Chandler 6-311G bases for first- and second-row atoms, respectively; and (iii) first-order spin–orbit corrections are explicitly taken into account. An assessment of the results obtained using such a procedure is presented. The results are also compared with corresponding all-electron calculations. We find that the G2[ECP] calculations give results which are generally comparable in accuracy to those of the G2[AE] calculations but which involve considerably lower computational cost. They are therefore potentially useful for larger bromine- and iodine-containing molecules for which G2[AE] calculations would not be feasible.