Structure and energetics of SiO2 polymorphs by quantum-mechanical and semiclassical approaches

Four SiO2 polymorphs (alpha-quartz, alpha-cristobalite, alpha-tridymite, an d coesite) hare been studied by periodic ab initio methods and by atomistic potentials. The former approach is based on all-electron localized basis s ets (atomic orbitals) and on three different Hamiltonians: Hartree-Fock(HF) , density functional theory in the local-density approximation (LDA), and d ensity functional theory including gradient corrections (B3-LYP). The semic lassical approach uses interatomic potentials parametrized either on empiri cal observables or on ab initio theoretical properties. Full structure opti mizations have been carried out, and phase transition energies derived; the results are compared with experimental data and with previous theoretical values obtained by plane-wave-pseudopotential techniques. HF and LDA struct ural results are slightly better than those for B3-LYP, whereas the order o f performance is reversed for the relative stability of polymorphs. The qua lity of semiclassical data is analyzed and discussed.