We have employed extensive configurational searches for numerical characterization of neutral (SiO 2 ) n clusters in their mechanically stable forms, i.e., at local potential energy minima. These calculations have used additive pair interactions derived by Tsuneyuki et al., supplemented with small-distance configurational constraints. A nearly complete catalog of structures and energies has been obtained for 1 e n e 4, and we also report the most stable clusters found for 5 e n e 8, as well as n ) 18. Also a few key sequences of metastable oligomers have been examined. Although the results illustrate the basic tendency toward tetrahedral coordination of silicons by oxygens, none of the stable clusters structurally resemble any of the known crystalline SiO 2 polymorphs. The n ) 18 results suggest that the surfaces of extended crystalline or glassy SiO 2 phases may exhibit anomalously coordinated silicons with formal valences +3 and +5.
Ab initio electronic structure calculations are reported for S4. Geometric and energetic parameters are calculated using the singles and doubles coupled-cluster method, including a perturbutional correction for connected triple excitation, CCSD(T), together with systematic sequences of correlation consistent basis sets extrapolated to the complete basis set limit. The geometry for the ground state singlet C2v structure of S4 is in good agreement with the microwave structure determined for S4. There is a low-lying D2h transition state at 1.6 kcal/mol which interchanges the long S-S bond. S4 has a low-lying triplet state (3B 1u) in D2h symmetry which is 10.8 kcal/mol above the C2v singlet ground state. The S-S bond dissociation energy for S4 into two S2(3Sigma*g) molecules is predicted to be 22.8 kcal mol(-1). The S-S bond energy to form S3+S(3P) is predicted to be 64 kcal/mol.
We examine prospects for metastability of six-coordinate high-pressure semiconductor phases at ambient temperature and pressure (STP). We investigate a simple “thermodynamic”, coherent transformation model for nanocrystals of size >2 nm and, as a quantitative example, apply it to the silicon β-tin to diamond structural phase transition. The unit cell transformation path is taken from a calculation by Mizushima et al. (Mizushima, K.; Yip, S.; Kaxiras, E. Phys. Rev. 1994, B50, 14952). Surface energies and the initial crystallite shape are included in an absolute rate model. The model assumes that the crystallite shape substantially changes to accommodate the unit cell c/a variation from 1.414 (diamond) to 0.55 (β-tin). The model predicts that the β-tin nanocrystal lifetime increases rapidly with increasing size. Near-round β-tin nanocrystals are more stable and have slower transformation rates than oblate spheroid nanocrystals with larger surface energy. For size >2 nm, both near-round and oblate β-tin nanocrystals are metastable with half-lives of years or more. An alternate, classical nucleation model is considered for surface nucleation in larger microcrystals. In this model the β-tin nanocrystal lifetime decreases with increasing size. Yet micron-size and smaller crystallites are metastable as well. Defect and strain-free β-tin microcrystals appear to be metastable at STP. More generally, stabilization of high-pressure semiconductor phases at STP should be more widespread in nanocrystals than in bulk crystals, because of (1) the relative ease in annealing out defects, strain, and impurities in nanocrystals and (2) the use of surface passivation in lowering the surface energy of the high-pressure phase.
Both variational Monte Carlo (VMC) and fixed-node diffusion MonteCarlo (DMC) are used to estimate the dissociation energy of Be 2 . The effect of using single-and multi-reference trial functions on the quality of the Monte Carlo estimates is investigated, with independent-particle wave functions ranging from a restricted Hartree-Fock (RHF) calculation up to a complete active space self-consistent field (CASSCF) with four valence electrons in 12 active orbitals. It was determined that the best trial function for DMC had a high cutoff for inclusion and included all 2s and 2p orbitals in a CASSCF(4,8) calculation. The best DMC estimate, D e ϭ 829(64) cm Ϫ1 , compares well with the experimental value of 839(10) cm Ϫ1 but is lower than values from the best basis-set calculations.
The quantum Monte Carlo (QMC) method is used to compute the atomization energy and the heat of formation of the propargyl radical, C3H3. The effective core potential and fixed-node approximations are used in the diffusion Monte Carlo (DMC) variant of QMC. Two generalized gradient approximation density functionals, B3LYP and B3PW91, are also applied for comparison. The atomization energy determined by these methods is 606.12 kcal/mol (B3LYP), 610.24 kcal/mol (B3PW91), and 607.6(0.6) (DMC). The latter compares favorably with separate measurements of 608.0(3.0) and 608.5(1.2) kcal/mol. The ΔHf298 determined by these methods is 84.03 kcal/mol (B3LYP), 79.91 kcal/mol (B3PW91), 82.5(0.6) (DMC), and two independent measurements yield values of 82.5(3.0) and 81.5(1.2) kcal/mol.
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