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Nonrelativistic clamped nuclei interaction energies for a pair of helium atoms have been computed using the Gaussian geminal implementation of the coupled cluster theory with single and double excitations (CCSD). Effects of triple and quadruple excitations were subsequently included employing the conventional orbital approach and very large augmented, correlation-consistent bases extended by sets of bond functions. Up to the coupled cluster doubles (CCD) level, the Gaussian geminal expansions provide nearly an order of magnitude better accuracy than orbital expansions even if the latter results are extrapolated. The recommended values of the helium dimer interaction energy are 292.54 ± 0.04 K, −11.009 ± 0.008 K, and −4.619 ± 0.007 K at the interatomic distances equal to 4.0, 5.6, and 7.0 bohr, respectively. The major contributions to the error estimates come from the orbital parts of the calculations beyond the CCSD level.

The convergence of NMR indirect spin-spin coupling constants with the extension of the basis set is analyzed, based on calculations carried out at the multicon®gurational self-consistent-®eld level for the HF and H 2 O systems. For the dominant and dicult Fermi-contact contribution, the standard correlationconsistent basis sets of electronic-structure theory are not suitable, lacking¯exibility in the core region. Improved but not satisfactory convergence of the couplings is observed when decontracting the s functions of the correlation-consistent cc-pVXZ basis sets for 2 6. Next, by systematically extending these basis sets with tight s functions, new sets are obtained that are suciently¯exible for accurate calculations of indirect nuclear spin-spin couplings, without sacri®cing the smooth convergence behavior of the correlation-consistent basis sets.

Helium dimer interaction energies, Eint, obtained recently using the Gaussian geminal implementation of the coupled cluster doubles (CCD) and singles and doubles (CCSD) theory, were employed to evaluate the performance of conventional orbital calculations applying the correlation-consistent polarized valence X-tuple zeta (cc-pVXZ) bases, with X ranging from 4 to 7, and very large sets of bond functions. We found that while the bond functions improve dramatically the convergence of the doubles and triples contribution to the interaction energy, these functions are inefficient or even counterproductive in predicting the effect of the single excitations and the small contribution beyond the CCSD(T) (CCSD model with noniterative account of triple excitations) level of electronic structure theory. We also found that bond functions are very effective in extrapolation techniques. Using simple two-point extrapolations based on the single-power laws X-2 and X-3 for the basis set truncation error, the Gaussian geminal CCSD result for Eint, equal to -9.150 ± 0.001 K at the equilibrium interatomic distance of R = 5.6 bohr, could be reproduced with an error of 2-3 mK. Linear extrapolation of the functional dependence of the CCSD energy on the value of the second-order Møller-Plesset energy and the use of the known accurate value of the latter leads to an even smaller error. Using these extrapolation techniques with basis sets up to doubly augmented septuple-zeta quality and containing large sets of bond functions, we estimated the contribution of triple excitations within the CCSD(T) model to be -1.535 ± 0.002 K, with the error bars reflecting the spread of extrapolated results. The contribution beyond the CCSD(T) model, estimated from full configuration interaction (FCI) calculations with up to 255 orbitals, amounts to -0.323 ± 0.005 K. Combining the Gaussian geminal value of the CCSD energy with the orbital estimations of the CCSD(T) and FCI contributions, we found that Eint = -11.008 ± 0.008 K. This value is consistent with recent high-level orbital computations (van Mourik T., Dunning T. H.: J. Chem. Phys. 1999, 111, 9246; Klopper W.: J. Chem. Phys. 2001, 115, 761) but has substantially tighter error bounds. It differs somewhat, however, from the value of -10.98 ± 0.02 K obtained recently from the "exact" quantum Monte Carlo calculations (Anderson J. B.: J. Chem. Phys. 2001, 115, 4546).

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By combining large basis and complete basis set (CBS) extrapolations of the coupled-cluster equilibrium geometry results with rovibrational and relativistic corrections, we demonstrate that it is possible to achieve near-quantitative accuracy for the NMR shielding constants in three group 15 trifluorides - NF3, PF3 and AsF3. These systems provide a rich test set for the calculation of dynamic electron correlation effects on NMR shielding constants. Basis sets as large as aug-cc-pCV6Z were employed, together with coupled-cluster expansion up to CCSDT, at the CCSD(T)/aug-cc-pCVTZ optimised geometries. The results of this work serve to highlight the application of state-of-the-art theoretical techniques which can be employed to guide and supplement NMR experimentation. Combining chemical shifts (either from experiment or high-level calculations) has also enabled a revised reference 19F NMR shielding constant for gas phase CFCl3 to be determined.

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