A PERIODIC AB-INITIO STUDY OF THE STRUCTURE AND RELATIVE STABILITY OFSILICA POLYMORPHS

The equilibrium geometry and total energy of four all-silica zeolite f rameworks, sodalite, chabazite, faujasite and edingtonite, have been o btained 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 hamiltonia n (Hartree-Fock, or with an a posteriori correlation correction; vario us local and gradient-corrected density functional methods and Becke's hybrid scheme) is discussed. The various methods provide a similar or der in the relative stabilities; large quantitative differences are ho wever observed, Hartree-Fock and LDA results being at the extremes. Th e combined use of force-field and ab initio schemes in the geometry op timization is discussed. (C) 1998 Elsevier Science B.V. All rights res erved.