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(a) Part of a silicate mineral framework containing Si and O atoms, with the SiO units viewed as polyhedra; (b) a rigid-unit model of the framework for calculation of RUMs. The polyhedra are considered as rigid units and their vertices are connected by harmonic constraints; (c) templates and constraints for geometric simulation. A tetrahedral template is centred on each Si atom. Vertex O atoms are tethered by harmonic constraints to template vertices.
