Abstract
We have explored the possibility of using extrapolation techniques to estimate the complete basis set limit of nuclear shieldings of small water clusters. Several density functionals (Becke-Lee-Yang-Parr (BLYP), Perdew-Burke-Ernzerhof (PBE), OPTX-Perdew-Burke-Ernzerhof (OPBE), Swart-Solà-Bickelhaupt functional with Grimme's Dispersion included (SSB-D) and wavefunction methods (Hartree–Fock, Møller-Plesser perturbation theory for electron correlation up to second order (MP2) and coupled cluster) were used in combination with Dunning's (augmented) correlation-consistent and Jensen's polarisation-consistent basis sets. Although density functionals show larger nuclear shieldings than coupled cluster methods for these water clusters, differences in chemical shifts are much smaller. A new extrapolation scheme based on a Gaussian exponential function is shown to achieve better results for reaching the basis set limiting values, but the exponential needed for it cannot be obtained a priori with good confidence. This renders a two-point extrapolation for prediction of the basis set limiting value impossible.
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