Ab initio calculation of electric-field-gradient tensors of forsterite

Ab initio band-structure calculations based on the density functional theory have been performed for forsterite to obtain, with a parameter-free model, electric-field gradients for all nuclei. Calculations based on the generalized gradient approximation yield the ratio of the largest components of the calculated electric-field gradients within 1% of the experimental value for 25Mg and within 2% of the experimental value for 17O. The absolute values differ by about 5%, depending on which nuclear quadrupole moment is used in the conversion. The asymmetry parameters are also in good agreement with experimental data. Values obtained with a gradient-corrected exchange-correlation potential are better than those based on the standard local-density approximation. The calculated angles between the principal axes of the quadrupole coupling tensor and the crystallographic axes agree very well with the experimental data. For the M1 site the maximum deviation is 1.8 [degrees], and for the M2 site the maximum deviation is 0.6 [degrees]. The current calculations allow an evaluation of three sets of experimental data for the 17O electric-field gradient. They also confirm a proposed assignment of the measured electric-field gradient tensors to specific O atoms.