Accurate quadrupolar NMR relaxation rates of aqueous cations from classical molecular dynamics

Nuclear magnetic resonance (NMR) relaxation rates encode information about the collective and local dynamics around nuclei. Provided a suitable microscopic model is available, this allows investigating, e.g., the solvation shell dynamics around aqueous ions. Previous attempts with molecular dynamics simulations faced the double challenge of calculating accurately the microscopic properties governing the relaxation process, such as the electric field gradient (EFG) at the nucleus, and of sampling the trajectories over sufficiently long times. Here we show how to compute the NMR relaxation rate from classical molecular dynamics simulations. We use a recently derived force field parametrized on ab initio calculations and show that the EFG predicted by this force field can be used to accurately estimate the one computed by DFT using the PAW method where the electronic structure is described explicitly. The predicted relaxation rates for aqueous alkaline and alkaline Earth cations are in good agreement with experimental data. Our approach opens the way to the quantitative interpretation of these rates with molecular simulation.