A b i n i t i o calculations on C2, Si2, and SiC

Abstract

Full configuration-interaction (FCI) calculations have been performed for the X 1Σ+g, a 3Πu, and b 3Σ−g states of C2 and the X 3Σ−g and A 3Πu states of Si2 in valence double-zeta plus polarization one-particle basis sets. Of the various approximate correlation treatments compared to the FCI, the CASSCF/MRCI method is found to be far superior in reproducing the FCI separations between the low-lying states. Hence this procedure was taken to near the one-particle basis set limit using [5s4p3d2f1g] carbon and [6s5p3d2f1g] silicon atom Gaussian basis sets contracted using the newly devised atomic natural orbital (ANO) method. The computed C2 separations in the large basis set are in excellent agreement with experiment. The ground state of Si2 is definitively determined to be X 3Σ−g, but the A 3Πu state is only 440±100 cm−1 higher in energy. For SiC, the A 3Σ− state is nearly 4000 cm−1 above the X 3Π ground state. Our best estimates of the spectroscopic parameters for the X 3Π state of SiC are re=1.719 Å, ωe=962 cm−1 and De=4.4 eV.

Communication: Chemical bonding in carbon dimer isovalent series from the natural orbital functional theory perspective

“…3, features three filled σ -type molecular orbitals and one bonding π molecular orbital, in agreement with previous CI results. 42 Furthermore, the EKT estimated principal ionization energy agrees satisfactorily with the experimental mark, as shown in Table I, which gives confidence on the reliability of our computed NOF orbitals.…”

“…15 The 1 + g state of Si 2 is indeed an excited state lying significantly above the 3 − u ground state. 42 Silicon dimer, in general, must have a very different electronic structure as compared to carbon dimer, as suggested by its very different chemical properties. The nature of the chemical bond of its 1 + g state is not an exception.…”

Ab initio spectroscopy and thermochemistry of the BN molecule

“…The T~ of only 247 cm-1 with the 6-311 G ( 2 d f ) and 120cm -1 with the [ 5 s 3 p 2 d l f ] basis set is almost an order of magnitude smaller than that for C2, and should thus be even more valuable as a benchmark for electron correlation methods than the C 2 separation commonly used (e.g. [47,48,49]) for this purpose.…”

High-temperature mass spectrometry: Instrumental techniques, ionization cross-sections, pressure measurements, and thermodynamic data (IUPAC Technical Report)

“…For gaseous molecules, the corresponding functions are generally calculated with statistical mechanical formulae [34,37,38,[236][237][238] from molecular parameters, i.e., geometry (interatomic distances, bond angles), vibrational frequencies, excitation energies, and multiplicities of electronic states [237,239], which are primarily deduced from infrared, Raman, or ultraviolet spectroscopic studies of gaseous or matrix-isolated species. In the last few decades, molecular data have been obtained increasingly from quantum-chemical calculations [240][241][242][243][244]. Structural data may also be derived from electron diffraction measurements in the gas phase.…”