EFGShield — A program for parsing and summarizing the results of electric field gradient and nuclear magnetic shielding tensor calculations

A theory for the ab initio calculation of all-electron NMR chemical shifts in insulators using pseudopotentials is presented. It is formulated for both finite and infinitely periodic systems and is based on an extension to the Projector Augmented Wave approach of Bloechl [P. E. Bloechl, Phys. Rev. B 50, 17953 (1994)] and the method of Mauri et al [F. Mauri, B.G. Pfrommer, and S.G. Louie, Phys. Rev. Lett. 77, 5300 (1996)]. The theory is successfully validated for molecules by comparison with a selection of quantum chemical results, and in periodic systems by comparison with plane-wave all-electron results for diamond.

17O NMR parameters, both the chemical shifts and the quadrupolar parameters, are calculated for SiO2 polymorphs using density functional theory with the generalized gradient-corrected PBE functional. The gauge including projector augmented wave (GIPAW) method (Pickard, C. J.; Mauri, F. Phys. Rev. B2001, 63, 245101) ensures the reproduction of all electron results while using computationally efficient pseudopotentials. The use of plane-waves permits fully converged calculations to be performed on structures containing 144 atoms in the unit cell, without the need to resort to the cluster approximation. The calculated NMR parameters of cristobalite, quartz, coesite, and faujasite are in excellent agreement with experimental data. This demonstrates that density functional theory is able to reproduce with high accuracy the 17O NMR parameters in SiO2 systems. This precision is used to assign the spectrum of the zeolite ferrierite. The data calculated for SiO2 are used to confirm that no simple correlation between the chemical shift and Cq NMR parameters and Si-O-Si angle exists, emphasizing the importance of predictive theories in this field.