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 mod elling approaches, are reported for the efg tensor at sodium in NaNO2. Calc ulations based on different-sized clusters are compared and it is shown tha t resonable agreement with experiment can be obtained for a symmetrical clu ster that extends beyond the immediate coordination environment of the sodi um cation. The accuracy of calculations based on this cluster are, however, very dependent upon basis set. Hybrid DFT methods were not found to give s ignificantly 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 ca lculations. HF methods were found to be superior to DFT methods in the appr oach used. It is suggested that periodic ab initio HF calculations using th e standard 6-21G basis set, with suitable basis set optimisation to take in to account the cationic nature of sodium, can provide a routine and consist ent method for predicting sodium efg tensor information for ionic sodium co mpounds. Such a method would be important for assignment purposes in materi als in which there are different crystallographic sites for the sodium cati on.