Highly accurate coupled-cluster singlet and triplet pair energies from explicitly correlated calculations in comparison with extrapolation techniques

Coupled-cluster singles and doubles (CCSD) valence shell correlation energies of the systems CH₂ (<¹A₁ state), H₂O, HF, N₂, CO, Ne, and F₂ are computed by means of standard calculations with correlation-consistent basis sets of the type cc-pVxZ (x = D, T, Q, 5, 6) and by means of explicitly correlated coupled-cluster calculations (CCSD-R12/B) with large uncontracted basis sets of the type 19s14p8d6f4g3h for C, N, O, F, and Ne and 9s6p4d3f for H. These CCSD-R12/B calculations provide reference values for the basis set limit of CCSD theory. The computed correlation energies are decomposed into singlet and triplet pair energies. It is established that the singlet pair energies converge as X^?3 and the triplet pair energies as X^?5 with the cardinal number of the correlation-consistent basis sets, and an extrapolation technique is proposed that takes into account their different convergence behaviour. Applied to the cc-pV5Z and cc-pV6Z results, this new extrapolation yields pair energies with a mean absolute deviation of 0.02 mE_h from the CCSD-R12/B reference values. For the seven systems under study, the extrapolated total valence shell correlation energies agree to within 0.2 mE_h with the CCSD-R12/B benchmark data.     

An accurate,global, ab initio potential energy surface for the H+ 3 molecule

A new global, ground-state, Born-Oppenheimer surface is presented for the H⁺ ₃ system. The energy switching approach has been used to combine different functional forms for three different regimes: a spectroscopic expansion at low energy, a Sorbie-Murrell function at high energy and known long-range terms combined with accurate diatomic potentials at large separations. At low energies we have used the ultra high accuracy ab initio data of Cencek et al. (1998, J. chem. Phys., 108, 2831). At intermediate energy we have calculated 134 new ab initio energies using a high accuracy, explicitly correlated procedure. The ab initio data of Schinke et al. (1980, J. chem. Phys., 72, 3909) has been used to constrain the high energy region. Two fits are presented which differ somewhat in their behaviour at energies over 45 000 cm^?1 above the H⁺ ₃ minimum. Below this energy, the fits reproduce each set of ab initio data close to their intrinsic accuracy. The ground state surface should provide a suitable starting point for renewed studies of the near-threshold photodissociation spectrum originally reported by Carrington et al. (1982, Molec. Phys., 45, 753).     

Anatomy of relativistic energy corrections in light molecular systems

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.     

EFFECTS OF in vitro EXPOSURE TO ULTRAVIOLET RADIATION ON THE FUNCTIONAL ACTIVITY OF LYMPHOCYTES, WITH EMPHASIS ON SUSCEPTIBILITY OF DIFFERENT SPECIES

Abstract In this study lymphocytes from blood and/or spleen of different species (rat, mouse, human) were exposed to different doses of ultraviolet radiation (UVR). The functional activity of these lymphocytes was determined using assays for mitogen proliferation and the mixed lymphocyte response (MLR). These experiments demonstrated that in vitro exposure to UVR causes a dose-dependent decrease of the MLR activity of the irradiated lymphocytes. Viability of lymphocytes and mitogen proliferation responses were also decreased by UVR exposure but less severe in comparison to the MLR. Lymphocytes of rats seem to be more sensitive to UVR as compared to lymphocytes of mice and humans.