RELEVANCE OF RELATIVISTIC EXCHANGE-CORRELATION FUNCTIONALS AND OF FINITE NUCLEI IN MOLECULAR DENSITY-FUNCTIONAL CALCULATIONS

Two types of relativistic corrections in molecular electronic structur e calculations that are based on the Kohn-Sham approach to density-fun ctional theory have been studied: (1) relativistic corrections to the exchange-correlation functional, both in the local density and in the generalized gradient approximation; (2) a description of the nuclear c harge distribution chat avoids the Coulomb singularity. The diatomics AuH, AuCl, Au-2, Ag-2, and Cu-2 were chosen as examples. Although sign ificant effects on the total energy and on core-level energies are fou nd, only the relativistic correction to local-density approximation ha s a noticeable impact on molecular observables: it induces changes of bond lengths by up to 0.005 Angstrom, of vibrational frequencies by up to 10 cm(-1), and of binding energies by up to 0.05 eV. Thus, taken t ogether, the relativistic corrections discussed here are much smaller than those obtained with density-gradient corrections to the exchange- correlation functional. Therefore, the common practice to neglect thes e relativistic corrections in molecular density-functional calculation s is justified, at least for compounds without superheavy elements.