Introducing Pseudoramps and Mixed Ramp-Gaussian Jensen Basis Sets for Better Nuclear Densities

Abstract

Gaussian basis sets dominate quantum chemistry but struggle to model near-core electron densities and thus nuclear magnetic resonance (NMR) spectral properties. Mixed ramp-Gaussian (RG) basis sets show significant promise for these core properties due to the inclusion of a ramp-function with a non-zero nuclear-electron cusp. To enable quicker testing of the potential of RG basis sets for core chemistry, here we approximate ramps as a large linear combination of Gaussians called pseudoramps, thus enabling standard quantum chemistry packages to be used to approximate RG basis set results. We produce and test rampified general-purpose segmented Jensen basis sets. These basis sets retain the valence chemistry of their parent all-Gaussian basis sets, as desired, but unfortunately fail to show significantly improved performance in core chemistry. Crucially, for NMR spin-spin couplings (the most promising potential application of RG basis sets), general-purpose basis sets are so poorly performing that results cannot be interpreted. For chemical shifts, P-ramps are likely required for improved performance. We conclude that the use of pseudoramps to test the performance of ramp-Gaussian basis sets is extremely helpful, decoupling methodology development and evaluation from implementation, but that more sophisticated basis set optimisation will be required to identify potential advantages of ramp-Gaussian basis sets over all-Gaussian basis sets.