Stationary Points on Potential Energy Surface of Cyclic C3H3 with Coupled-Cluster Approaches and Density Functional Theory.

Abstract

Cyclic C3H3 is the simplest cyclic hydrocarbon, but its configuration is complicated. The ground 2E″ state at the equilateral triangle geometry with D3h symmetry undergoes both Jahn-Teller (JT) distortion and pseudo-Jahn-Teller (PJT) distortion to structures with C3v, C2v, Cs, or C2 symmetry. Previous works using complete active space self-consistent field (CASSCF) or multiconfiguration SCF (MCSCF) differ on the characteristics of these structures. To clarify the characteristics of these stationary points on the potential energy surface (PES) of this radical, coupled-cluster methods CCSD, CCSD(T), and EOMEA/IP-CCSD as well as density functional theory are employed to calculate their geometries, harmonic frequencies, and relative energies. Deformations between these stationary points due to JT and PJT effects are analyzed in detail. Most of the results with these methods are consistent with each other, except for the b2 mode of the 2B1 state with C2v symmetry. CCSD and CCSD(T) provide an imaginary frequency for this vibrational mode, while it is calculated to be real with the other methods as well as with EOM-CCSDT-3. This may be related to unstable reference in CCSD and CCSD(T) calculations. On the other hand, most of the employed exchange-correlation functionals provide reliable results on the characteristics of these stationary points. Our results show that the 2A' state with Cs symmetry is the only minimum structure on the PES of cyclic C3H3. The 2A2 and 2B1 states of the C2v structure are second-order saddle points, while both the 2A state of the C2 structure and the 2A″ state of the Cs structure are transition states connecting the global minimum 2A' state.