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Abstract: Motivated by the theoretical study of Saito and Ono (2011) on three crystalline forms of SiO2 under pressure, quantum-chemical calculations on various free-space clusters of and for are reported here. Both neutral and cationic clusters have been examined, for both geometry and equilibrium bond lengths. Coupled clusters and correlation-corrected MP2 calculations are presented. For the cations, we emphasize especially the structural distortions occurring in removing degeneracies. [Copyright &y& Elsevier]

Quantum-chemical calculations are first reported on the building block Li2C2. Two low-lying isomers are predicted, and hence the equilibrium ground-state geometry by the coupled cluster method (CCSD). The ground-state is non-linear, contrasting with the other planar isomer. The dimer (Li2C2)2 is then examined by the Hartree–Fock plus second-order Møller–Plesset (HF+MP2) method. The ground-state geometry is predicted to have C4 on a linear chain. As to the trimer, the C6 atoms are predicted at the same ab initio level to lie on a hexagon. But the Li atoms prefer each to bond to two C atoms, rather than reproduce the geometry of benzene. Finally, some contact is made with a recent density functional theory study of solid lithium carbide under pressure. [ABSTRACT FROM PUBLISHER]