Improved virtual orbital multireference Mo\ller–Plesset study of the ground and excited electronic states of protonated acetylene, C2H3+.

The ground state geometries and associated normal mode frequencies of the classical and nonclassical protonated acetylene ion, i.e., the vinyl cation C2H3+, are computed using the complete active space self-consistent field and improved virtual orbital (IVO) complete active space configuration interaction methods. In addition, the minimum-energy reaction path for the classical to nonclassical interconversion is determined (as are excitation energies) using the IVO modification of multireference Mo\ller–Plesset (MRMP) perturbation theory. The IVO-MRMP treatment predicts the nonclassical structure to be 4.8 kcal/mol more stable than the classical one, which is consistent with other high level theoretical estimates. The proton affinity of acetylene from the IVO-MRMP treatment (154.8 kcal/mol) also agrees well with experiment (153.3 kcal/mol) and with earlier CASPT2 calculations (154.8 kcal/mol). We further report geometries and vibrational frequencies of low lying excited states of C2H3+, which have not been observed and/or studied before. Comparisons with previous highly correlated calculations further demonstrate the computational efficiency of the IVO-MRPT methods. [ABSTRACT FROM AUTHOR]