MODIFIED ELECTRON-GAS MODELINGS OF CALCITE AND ARAGONITE - COMPARISONOF POLARIZABLE ANION AND FULLY IONIC METHODS

Modified electron gas (MEG) methods have yielded crystal structures, l attice energies, and bulk moduli in close agreement with experiment wh en used to model ionic solids, but, when these methods were applied to covalently bonded substances, the agreement between the model and exp eriment was not as close. The use of a shell model for anion polarizat ion has improved agreement with experiment in the covalently bonded su bstances but has degraded such agreement in the ionic solids. These ob servations raise the question as to how well substances with both ioni c and covalent bonds would be modeled using MEG methods, with and with out allowances for anion polarization. Purely ionic modelings of calci te and aragonite, which disregard anion polarization, result in severa l significant errors. In the current work, we model these substances u sing polarizable anion MEG methods and obtain improved agreement with experiment for most, but not all, structure and energy parameters. Fur ther improvements were obtained by using the polarizable ion approach in conjunction with the use of fixed CO3 group geometries. We obtained a picture of the calcite-aragonite phase transition using the polariz able ion treatment that is an improvement over the purely ionic method . Our prediction for the transition pressure is still too high (100 kb ar compared with the experimental value of 3 kbar), but the difference can be mainly attributed to the fact that the model calcite equilibri um cell volume is too small.