Abstract
The projector-augmented wave (PAW) method is used to calculate electric field gradients (EFG) for various PAW potentials. A variety of crystals containing reactive nonmetal, simple metal, and transition elements, are evaluated in order to determine the predictive ability of the PAW method for the determination of nuclear quadrupole resonance frequencies in previously unstudied materials and their polymorphs. All results were compared to experimental results and, where possible, to previous density functional theory (DFT) calculations. The EFG at the 14N site of NaNO2 is calculated by DFT for the first time. The reactive nonmetal elements were not very sensitive to the variation in PAW potentials, and calculations were quite close to experimental values. For the other elements, the various PAW potentials led to a clear spread in EFG values, with no one universal potential emerging. Within the spread, there was agreement with other ab initio models.
Highlights
Nuclear quadrupole resonance (NQR) spectroscopy is highly specific to the material as the frequencies directly depend on the intrinsic electric field gradient (EFG) at the nuclear site [1]
Density functional theory (DFT) can be used to calculate the electric field gradient at the nucleus [15,16,17,18,19,20] and the NQR frequencies, including those for different polymorphs [3,21]. In principle this can greatly reduce the search time for new NQR transitions, but in practice is limited by the quality of predictions that can be produced with DFT
For each compound we swept through all combinations of potentials and computed the EFG for each potential configuration, which we converged with respect to the number of k-points and the plane-wave energy cutoff
Summary
Nuclear quadrupole resonance (NQR) spectroscopy is highly specific to the material as the frequencies directly depend on the intrinsic electric field gradient (EFG) at the nuclear site [1].One strong hallmark of this specificity is the ability of NQR to distinguish between polymorphs of a crystal [2,3,4]. Density functional theory (DFT) can be used to calculate the electric field gradient at the nucleus [15,16,17,18,19,20] and the NQR frequencies, including those for different polymorphs [3,21]. Most DFT calculations of the EFG in different bulk-like materials have either been full-potential linearized augmented plane wave (FP-LAPW) calculations or employed the projector-augmented wave (PAW) method [21]. While both methods use a plane wave expansion for the electronic wave function, they differ in how they treat the electric potential generated by the nuclei. The full potential method does not alter the potential in any way while the PAW method instead uses a pseudopotential for the potential around the nuclei
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