Ab initio many-body investigation of structure and stability of two-fold rings in silicates.

In this paper we present ab initio many-body calculations on the strain energy of W silica, taken as a model system for edge-sharing tetrahedral SiO(2) systems with respect to corner-sharing ones as in alpha quartz. The mean-field results were obtained using the restricted Hartree-Fock approach, while the many-body effects were taken into account by the second-order Møller-Plesset perturbation theory and the coupled-cluster approach. Correlation contributions are found to play an important role to determine the stability of edge-sharing units. The most sophisticated method used in our calculation, i.e., the coupled-cluster approach with single and double excitations, yields a strain energy of 0.0427 a.u. per Si(2)O(4) unit with respect to alpha quartz, which is even smaller than the value obtained by a previous density functional theory calculation.