Toward resolution of the silicon dicarbide (SiC2) saga: Ab initio excursions in the web of polytopism

The long-standing problem of the topography, energetics, and vibrational dynamics of the ground-state surface of SiC2 is systematically investigated by means of the gamut of state-of-the-art electronic structure methods, including single-reference correlation techniques as extensive as the coupled-cluster singles and doubles method augmented by a perturbative triples term [CCSD(T)], the Brueckner doubles method (ED) with analogous contributions from both triple and quadruple excitations [BD(TQ)], and second- through fifth-order Moller-Plesset perturbation theory (MP2-MP5), as well as the multiconfigurational complete-active-space self-consistent-field [CASSCF(12,12)] approach. The one-particle basis sets for these studies ranged from Si[6s4p1d], C[4s2y1d] to Si[7s6p4d3f2g1h], C[6s5p4d3f2g1h]. The methodological analysis resolves the polytopism problem regarding the mercurial potential energy surface for the circumnavigation of Si+ about C-2(-) in silicon dicarbide, whose topography is shown to exhibit almost all conceivable variations with level of theory. It is concluded that the (X) over tilde (1)A(1), global minimum of SiC2 is a T-shaped (C-2 upsilon) structure connected monotonically to a linear transition state 5.8 kcal mol(-1) higher in energy, thus ruling out any metastable linear isomer. Previously undocumented bent transition states and L-shaped minima are encountered at relatively high levels of theory, but ultimately these stationary points are shown to be spurious. High-level focal-point thermochemical analyses yield D-0(Si-C-2)=151 kcal mol(-1), and hence a substantial revision is made in the heat of formation, viz., Delta H(f,0)degrees(SiC2)=+155 kcal mol(-1). A complete quartic force field about the T-shaped minimum is determined at the CCSD(T) level with the aug-cc-pVTZ (Si[6s5p3d2f], C[5s4p3d2f]) basis set and then employed in a preliminary probe of contours for large-amplitude motion, anharmonicity of the vibrations, and zero-point effects on the molecular structure. (C) 1997 American Institute of Physics.