Large scale contracted MC–CI calculations on acetylene and its dissociation into two CH(2Π) radicals

Large scale MC–CI calculations with up to 178 000 configurations have been performed on acetylene and its dissociation into two CH(2Π) radicals with a newly developed contracted CI scheme. The geometry obtained for acetylene was RC–C = 1.208 Å (exptl. = 1.203 Å) and RC–H = 1.061 Å (1.060 Å). The dissociation energy into two CH(2Π) was De = 10.00 eV (10.26 eV). The barrier along the linear dissociation path recently predicted by Raimondi et al. was confirmed but their value of 0.33 eV was raised to 0.57 eV and the location of the barrier was moved from 6.0 to 5.0 a.u. The origin of the barrier is an avoided crossing between the states dissociating into two CH(2Π)—and two CH(4Σ−)—radicals. The energy difference between the states at the point of the avoided crossing was computed to be 0.19 eV from second root contracted CI calculations. The energy barrier was increased in going from MCSCF to CI and this unusual behavior is explained by the much larger amount of correlation energy in CH(2Π) than in CH(4Σ−). The minimum energy path is however found to be nonlinear and has no energy barrier. A simple molecular orbital argument is given for why this should be so. The potential surface for acetylene is further found to exhibit irregular regions with double minima for bending which were not predicted in the surface given recently by Carter et al. All the presently performed calculations were done on a minicomputer VAX-11/780.