Ab initio study of cluster molecules relevant to materials science and astrophysics: Development of combined bond-polarization basis sets for the accurate ab initio calculation of dissociation energies

The current state (December 1990) of ab initio theory is reviewed, with special emphasis on electron correlation methods.; A new family of Gaussian basis sets, the 'combined bond-polarization' basis sets, is introduced. Relative to common large basis sets, binding energies can be computed either with comparable accuracy ({dollar}pm{dollar}2 kcal/mol) at much reduced computational cost, or with much greater accuracy ({dollar}pm{dollar}0.12 kcal/mol per bond) at comparable cost. Several issues relating to basis set superposition error are discussed.; Subsequently, a number of cluster molecules relevant in materials science and astrophysics are studied. For B{dollar}sb3{dollar}, B{dollar}sb2{dollar}N, and BN{dollar}sb2{dollar}, the first published data whatsoever are presented. A literature disagreement on the dissociation energy of N{dollar}sb3{dollar} is resolved by large-scale ab initio calculations. Accurate spectroscopic constants for the lowest few states of SiC and BN are obtained, as well as JANAF-style thermodynamic functions up to 6000 K. An ad hoc reparametrization of MNDO for C{dollar}sb{lcub}rm n{rcub}{dollar} clusters leads to much improved results, including very accurate harmonic frequencies. A comparison of computed frequencies, infrared intensities, and isotope shifts--both ab initio and ad hoc MNDO--with available infrared data of argon-trapped carbon vapor leads to a completely revised assignment of these latter experimental frequencies. The latter has been instrumental in the experimental identification and characterization of C{dollar}sb4{dollar},C{dollar}sb5{dollar},C{dollar}sb7{dollar}, and C{dollar}sb9{dollar}. The ground state of C{dollar}sb4{dollar}, a long-standing issue in the literature, has been shown to be cyclic by ab initio calculations using large basis sets; an error in the experimental heat of formation has also been corrected. A definitive characterization of the C{dollar}sb3sp{lcub}+{rcub}{dollar} ion, a 'problem molecule' in quantum chemistry, has been given using a variety of single- and multi-reference correlation treatments.; It is also shown that single excitations have an unusually large effect on the geometry and harmonic frequency of spin-contaminated radicals. Finally, a critical analysis, both theoretically and numerically, of size-consistency correction for limited configuration interaction has been made.