The error due to neglecting the core spin-orbit splitting in valence ZORA calculations with frozen core approximation and its elimination

In valence zeroth-order regular approximation (ZORA) calculations with froz en core approximation, when the basis set optimized to the related scalar r elativistic ZORA calculations is used, neglecting the core spin-orbit split ting may result in additional basis set truncation errors. It is found that the error is negligible for most elements except the 6p-block elements. Wh en the basis set is extended by a p-type STO function put on the 6p element atoms with the zeta value proper to 5p(1/2) orbitals, the error can be red uced to be negligible. The calculated atomic properties related to valence orbitals can be improved greatly by use of this extended basis set. The fro zen core approximation calculations of some molecules containing Tl, Pb and Bi with closed shells show that neglecting the core spin-orbit splitting o nly slightly affects the calculated bond lengths and bond energies, and the calculated molecular property can also be improved slightly by use of the extended basis sets. (C) 2001 Published by Elsevier Science B.V. All rights reserved.