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 vibration al dynamics of the ground-state surface of SiC2 is systematically inve stigated by means of the gamut of state-of-the-art electronic structur e methods, including single-reference correlation techniques as extens ive as the coupled-cluster singles and doubles method augmented by a p erturbative triples term [CCSD(T)], the Brueckner doubles method (ED) with analogous contributions from both triple and quadruple excitation s [BD(TQ)], and second- through fifth-order Moller-Plesset perturbatio n theory (MP2-MP5), as well as the multiconfigurational complete-activ e-space self-consistent-field [CASSCF(12,12)] approach. The one-partic le basis sets for these studies ranged from Si[6s4p1d], C[4s2y1d] to S i[7s6p4d3f2g1h], C[6s5p4d3f2g1h]. The methodological analysis resolves the polytopism problem regarding the mercurial potential energy surfa ce 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 connect ed monotonically to a linear transition state 5.8 kcal mol(-1) higher in energy, thus ruling out any metastable linear isomer. Previously un documented bent transition states and L-shaped minima are encountered at relatively high levels of theory, but ultimately these stationary p oints 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 r evision is made in the heat of formation, viz., Delta H(f,0)degrees(Si C2)=+155 kcal mol(-1). A complete quartic force field about the T-shap ed 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 vib rations, and zero-point effects on the molecular structure. (C) 1997 A merican Institute of Physics.