How reliable is the theoretical structure of SiC 2?

Abstract

The effects of basis set choice and level of inclusion of correlation on the prediction of the energy difference between the linear and triangular forms of SiC 2 are discussed. It is concluded that at the SCF level the ordering is dependent on basis set choice and that in fact, contrary to earlier belief, a carefully optimized basis set may predict the triangular form to be slightly more stable even at the SCF level. However, correlation is essential to ensure the expected energy difference. Improvement of the basis set causes opposing changes in the absolute SCF and correlation energy difference between the two forms such that the total energy difference remains small. Estimating total energies by adding correlation effects calculated from small basis sets to results of largebasis SCF calculation is thus not valid. The energy difference between the linear and the triangular forms is 3.74 mh (2.35 kcal/mole) in MBPT (4) using the largest, 120 CGTO basis set. It is argued that inclusion of higher-order correlation may lead to further stabilization of the triangular form.