An ab initio two-component relativistic method including spin-orbit coupling using the regular approximation

In this paper we present the implementation of the two-component scaled zeroth-order regular approximation (ZORA) method in the molecular electronic structure package GAMESS-UK. It is the first application of this method, which was earlier investigated in the context of density functional theory, in molecular ab initio basis set calculations. The performance of the method is tested in atomic calculations, which we can compare with numerical results, on xenon and radon and in molecular calculations on the molecules AgH, HI, I-2, AuH, TlH, and Bi-2. In calculations on the I-2 molecule we investigated the effect of the different approaches regarding the internal Coulomb matrix used in the ZORA method. For the remaining molecules we computed harmonic frequencies and bond lengths. It is shown that the scaled ZORA approach is a cost-effective alternative to the Dirac-Fock method. (C) 2000 American Institute of Physics. [S0021-9606(00)32034-7].