Title: Surveying the Ground Electronic State Potential Energy Surface of the “Mysterious” CO dimer

Authors: M. Bosquez, P. S. Żuchowski, and A. G. Császár

DOI: 10.1080/00268976.2026.2628252

Abstract: Eleven feasible stationary points have been located on the potential energy surface of the ground electronic state of (CO)2 with shapes H, I , T, V, X, and Z. The global minimum has a planar, slipped, antiparallel arrangement of the atoms with a CC contact. There is a secondary minimum with an OO contact. The first-order transition state connecting the two minima has a CO contact. The remaining feasible stationary points are higher-order transition states. At intermediate levels of electronic structure theory, one can easily become lost in the web of polytopism. The relative energies of the most important stationary points were determined with the help of the focal-point analysis scheme. The relative energies are based on calculations up to the CCSDT(Q) level of electronic structure theory, basis sets up to aug-cc-pV6Z, and the inclusion of so-called ‘small’ corrections due to core-core and core-valence correlation, relativistic effects, and the diagonal Born–Oppenheimer correction. The electronic interaction energy of the global minimum is −(hc)136.8(15) cm−1, while the electronic energy difference between the global and local minima is (hc)17.1(25) cm−1. A detailed interaction energy analysis via symmetry-adapted perturbation theory suggests that the bonding in the dimer arises primarily from dispersion rather than electrostatics.