Anatomy of relativistic energy corrections in light molecular systems |
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Authors: | GYÖRGY TARCZAY ATTILA G. CSÁSZÁR WIM KLOPPER HARRY M. QUINEY |
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Affiliation: | 1. Department of Theoretical Chemistry , E?tv?s University , P.O. Box 32, H-1518, Budapest, 112, Hungary;2. Theoretical Chemistry Group, Debye Institute, Utrecht University , Padualaan 14, NL-3584, CH Utrecht, The Netherlands;3. School of Chemistry, University of Melbourne , Parkville, Victoria, 3010, Australia |
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Abstract: | Relativistic energy corrections which arise from the use of the Dirac-Coulomb Hamiltonian, and the Gaunt and Breit interaction operators, plus Lamb-shift effects have been determined for the global minima of the ground electronic states of C2H6, NH3, H2O, [H,C,N], HNCO, HCOOH, SiC2, SiH? 3, and H2S, and for barrier characteristics for these molecular systems (inversion barrier of NH3 and SiH? 3, barrier to linearity of H2O, H2S, and HNCO, rotational barrier of C2H6, difference between conformations of HCOOH (Z/E) and SiC2 (linear/T-shaped), and isomerization barrier of HCN/HNC). The relativistic calculations performed at the Hartree-Fock and the highly correlated CCSD(T) levels employed a wide variety of basis sets. Comparison of the perturbational and the four-component fully variational results indicate that the Coulomb-Pauli Hamiltonian and the lowest order Hamiltonian of direct perturbation theory (DPT(2)) are highly successful for treating the relativistic energy effects in light molecular systems both at a single point on the potential energy hypersurface and along the surface. Electron correlation contributions to the relativistic corrections are relatively small for the systems studied, and are comparable with the 2-electron Darwin correction. Corrections beyond the Dirac-Coulomb treatment are usually rather small, but may become important for high accuracy ab initio calculations. |
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