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 (BD) with analogous contributions from both triple and quadruple excitations [BD(TQ)], and second-through fifth-order Mo/ller–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 $Si[6s4p1d],$ $C[4s2p1d]$ to $Si[7s6p4d3f2g1h],$ $C[6s5p4d3f2g1h].$ The methodological analysis resolves the polytopism problem regarding the mercurial potential energy surface for the circumnavigation of $Si+$ about $C2−$ in silicon dicarbide, whose topography is shown to exhibit almost all conceivable variations with level of theory. It is concluded that the $X̃ 1A1$ global minimum of $SiC2$ is a T-shaped $(C2v)$ 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 $D0(Si–C2)=151 kcal mol−1,$ and hence a substantial revision is made in the heat of formation, viz., $ΔHf,0∘(SiC2)=+155 kcal mol−1.$ 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.