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We apply density functional theory (DFT) to the calculation of the (17)O NMR parameters in Ca and Mg oxides and aluminosilicates. We study the accuracy of the Perdew, Burke, and Ernzerhof (PBE) generalized-gradient approximation to DFT in the description of these systems and the origin of the experimentally observed large dependence of the (17)O chemical shift on the alkaline earth ion. We find that (i) the partially covalent nature of the Ca-O bond has a huge impact on the O chemical shifts. The Ca-O covalence alone explains why in Ca oxides and aluminosilicates the (17)O chemical shifts are much more deshielded than those of the corresponding Mg compounds. (ii) The Ca-O covalence is overestimated by the PBE functional. Thus PBE-DFT is not able to reproduce the measured (17)O NMR parameters in Ca oxide and Ca aluminosilicates. (iii) It is possible to correct for the PBE-DFT deficiency in a simple and transferable way and to predict very accurate (17)O NMR parameters. Such accuracy allows us to assign the (17)O NMR spectra of two important model systems: the grossite aluminate (CaAl(4)O(7)) and the wollastonite (CaSiO(3)) silicate.

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We have computed the (17)O NMR parameters of an amorphous calcium aluminosilicate (CAS) from first-principles. The atomic coordinates of a CAS glass of composition (CaO)(0.21)(Al(2)O (3))(0.12)(SiO (2))(0.67) were obtained by quenching a liquid to room temperature by the means of ab initio molecular-dynamics simulations of the Car-Parrinello type. The structure of the glass is found to be overall in good agreement with diffraction experiments. Some excess nonbridging O (NBO) atoms are found and are compensated by tricluster O atoms, i.e., by 3-fold coordinated O atoms to 4-fold coordinated Al or Si atoms. The glass coordinates were used to compute the (17)O NMR parameters using GGA-DFT and a correction of the Ca 3d orbital energy. The chemical shifts and the electric field gradients were obtained with the gauge including projector augmented-wave (GIPAW) and the projector augmented-wave (PAW) methods, respectively. The simulated 2D-3QMAS NMR spectrum of the CAS glass is in very good agreement with the available experimental data, notably because it takes into account the disorder present in the glass. This agreement further validates our CAS glass model. We show that the oxygen triclusters are not visible in a 2D-3QMAS NMR (17)O spectrum since their NMR parameters overlap with those of the Al-O-Si, Si-O-Si, or Al-O-Al sites. Finally, correlations between the structural characteristics and the values of the NMR parameters are extracted from the calculation with the aim of helping the interpretation of NMR spectra of glasses of similar compositions.

We study a calcium aluminosilicate glass of composition (SiO 2 ) 0.67 -(Al 2 O 3 ) 0.12 -(CaO) 0.21 by means of molecular-dynamics (MD) simulations, using a potential made of two-body and threebody interactions. In order to prepare small samples that can subsequently be studied by firstprinciples, the finite size effects on the liquid dynamics and on the glass structural properties are investigated. We find that finite size effects affect the Si-O-Si and Si-O-Al angular distributions, the first peaks of the Si-O, Al-O and Ca-O pair correlation functions, the Ca coordination and the oxygen atoms environment in the smallest system (100 atoms). We give evidence that these finite size effects can be directly attributed to the use of three-body interactions.

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