Ab initio study of the c3+ cation using multireference methods

Abstract

The energy difference between the linear 2-SIGMA-u+ and cyclic B-2(2) structures of C3+ has been investigated by using large [5s3p2dlf] basis sets and multireference electron correlation treatments, including complete active space SCF (CASSCF), multireference CI (MRCI), and averaged coupled-pair functional (ACPF) methods, as well as the single-reference quadratic configuration interaction [QCISD(T)] method. Our best estimate, including a correction for basis set incompleteness, is that the linear form lies above the cyclic form by 5.2(1.0)+1.5 kcal/mol. The 2-SIGMA-u+ state is probably not a transition state but a local minimum. Reliable computation of the cyclic/linear energy difference in C3+ is extremely demanding of the electron correlation treatment used: of the single-reference methods previously considered, CCSD(T) and QCISD(T) perform best. The MRCI+Q-(0.01)/[4s2pld] energy separation of 1.68 kcal/mol should provide a comparison standard for other electron correlation methods applied to this system.