Validating orbital-optimized linearized coupled-cluster theory for high-accuracy thermochemistry and spectroscopy of...

High-level ab initio estimates for the equilibrium structure (rCC = 1.330 79 ± 0.000 31 Å, rCH = 1.080 75 ± 0.000 21 Å, and αCCH = 121.423° ± 0.023°), the adiabatic ionization energy (10.52 eV), and the 0 K proton affinity (673.2 kJ/mol) of ethylene were obtained via focal-point extrapolation of coupled cluster results to the basis set limit. This methodology also provided accurate equilibrium structures for the ethylene radical cation (ionized ethylene) and the ethenium cation (bridged protonated ethylene). An additive correction scheme allowed for the prediction of the vibrational spectrum of ethylene with a root-mean-square error of 2.4 cm-1. A systematic evaluation of popular wavefunction-based methods revealed that the recently implemented orbital-optimized linearized coupled cluster (OLCCD) method predicts the geometry and properties (the ionization potential, the proton affinity, and the fundamental frequencies) of ethylene significantly better than the orbital-optimized Møller-Plesset (OMP2 and OMP3) and the standard coupled cluster with singles and doubles methods. Notably, where coupled-cluster with single, double, and perturbative triple excitations analytic Hessians suffer from wavefunction instability issues along several normal modes, OLCCD yields superior vibrational anharmonicities, making it a robust alternative for challenging π-systems.