Anatomy of relativistic energy corrections in light molecular systems

Relativistic energy corrections which arise from the use of the Dirac-Coulo mb Hamiltonian, and the Gaunt and Breit interaction operators, plus Lamb-sh ift effects have been determined for the global minima of the ground electr onic states of C2H6, NH3, H2O, [H, C, N], HNCO, HCOOH, SiC2, SiH3-, and H2S , and for barrier characteristics for these molecular systems (inversion ba rrier of NH3 and SiH3-, barrier to linearity of H2O, H2S, and HNCO, rotatio nal barrier of C2H6, difference between conformations of HCOOH (Z/E) and Si C2 (linear/T-shaped), and isomerization barrier of HCN/HNC). The relativist ic calculations performed at the Hartree-Fock and the highly correlated CCS D(T) levels employed a wide variety of basis sets. Comparison of the pertur bational and the four-component fully variational results indicate that the Coulomb-Pauli Hamiltonian and the lowest order Hamiltonian of direct pertu rbation theory (DPT(2)) are highly successful for treating the relativistic energy effects in light molecular systems both at a single point on the po tential energy hypersurface and along the surface. Electron correlation con tributions to the relativistic corrections are relatively small for the sys tems studied, and are comparable with the 2-electron Darwin correction. Cor rections beyond the Dirac-Coulomb treatment are usually rather small, but m ay become important for high accuracy ab initio calculations.