A b i n i t i o effective core potentials for molecular calculations. II. All-electron comparisons and modifications of the procedure

Recently methods have been developed L. R. Kahn, P. Baybutt, and D. G. Truhlar, J. Chem. Phys. 65, 3826 (1976) to replace the core electrons of atoms by ab initio effective core potentials (ECP). Valence-electron calculations are reported on the ground states of HF, F2, HCl, LiCl, and Cl2 using ECP’s for F and Cl, and the results are compared to all-electron calculations. Significant discrepancies are found (as much as 20 kcal/mol for De and 0.3 bohr for Re in the case of Cl2) between the two sets of calculations. The original formulation of generating ECP’s can lead to artificial attractive tails in the potential at large values of r which leads to potential energy curves which are too attractive. The procedure of generating ECP’s is modified to remove the long-range attractive tails. Excellent agreement between the valence-electron and all-electron calculations is then obtained.