A comparison of ab initio cluster and periodic calculations of the electric field gradient at sodium in NaNO2

Ab initio HF and DFT calculations, based upon both cluster and periodic modelling approaches, are reported for the efg tensor at sodium in NaNO2. Calculations based on different-sized clusters are compared and it is shown that resonable agreement with experiment can be obtained for a symmetrical cluster that extends beyond the immediate coordination environment of the sodium cation. The accuracy of calculations based on this cluster are, however, very dependent upon basis set. Hybrid DFT methods were not found to give significantly better results than HF methods. The periodic calculations are shown to give very good agreement with experiment for both a mid-sized, as well as a smaller standard 6-21G, basis set. In both cases the presence of d-polarisation functions on Na and O is essential to the accuracy of the calculations. HF methods were found to be superior to DFT methods in the approach used. It is suggested that periodic ab initio HF calculations using the standard 6-21G basis set, with suitable basis set optimisation to take into account the cationic nature of sodium, can provide a routine and consistent method for predicting sodium efg tensor information for ionic sodium compounds. Such a method would be important for assignment purposes in materials in which there are different crystallographic sites for the sodium cation.