A periodic ab initio study of the structure and relative stability of silica polymorphs

The equilibrium geometry and total energy of four all-silica zeolite frameworks, sodalite, chabazite, faujasite and edingtonite, have been obtained at a periodic ab initio all-electron level (CRYSTAL code) and compared with the corresponding quantities for alpha- and beta-quartz. The dependence of the results on the adopted basis set and hamiltonian (Hartree-Fock, or with an a posteriori correlation correction; various local and gradient-corrected density functional methods and Becke's hybrid scheme) is discussed. The various methods provide a similar order in the relative stabilities; large quantitative differences are however observed, Hartree-Fock and LDA results being at the extremes. The combined use of force-field and ab initio schemes in the geometry optimization is discussed. (C) 1998 Elsevier Science B.V. All rights reserved.