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 zer oth-order regular approximation (ZORA) method in the molecular electronic s tructure package GAMESS-UK. It is the first application of this method, whi ch 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 resul ts, 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 investi gated the effect of the different approaches regarding the internal Coulomb matrix used in the ZORA method. For the remaining molecules we computed ha rmonic frequencies and bond lengths. It is shown that the scaled ZORA appro ach is a cost-effective alternative to the Dirac-Fock method. (C) 2000 Amer ican Institute of Physics. [S0021-9606(00)32034-7].