We present an extensive study of the four-dimensional potential energy surface (4D-PES) of the carbon dioxide dimer, (CO2)2. This PES is developed over the set of intermolecular coordinates. The electronic computations are carried out at the explicitly correlated coupled cluster method with single, double, and perturbative triple excitations [CCSD(T)-F12] level of theory in connection with the augmented correlation-consistent aug-cc-pVTZ basis set. An analytic representation of the 4D-PES is derived. Our extensive calculations confirm that “Slipped Parallel” is the most stable form and that the T-shaped structure corresponds to a transition state. Later on, this PES is employed for the calculations of the vibrational energy levels of the dimer. Moreover, the temperature dependence of the dimer second virial coefficient and of the first spectral moment of rototranslational collision-induced absorption spectrum is derived. For both quantities, a good agreement is found between our values and the experimental data for a wide range of temperatures. This attests to the high quality of our PES. Generally, our PES and results can be used for modeling CO2 supercritical fluidity and examination of its role in planetary atmospheres. It can be also incorporated into dynamical computations of CO2 capture and sequestration. This allows deep understanding, at the microscopic level, of these processes.
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A four-dimensional intermolecular potential energy surface for the carbon dioxide dimer has been computed using the many-body symmetry-adapted perturbation theory (SAPT) and a large 5s3p2d1f basis set including bond functions. The SAPT level applied is approximately equivalent to the supermolecular many-body perturbation theory at the second-order level. An accurate fit to the computed data has been obtained in a form of an angular expansion incorporating the asymptotic coefficients computed ab initio at the level consistent with the applied SAPT theory. A simpler site-site fit has also been developed to facilitate the use of the potential in molecular dynamics and Monte Carlo simulations. The quality of the new potential has been tested by computing the values of the second virial coefficient which agree very well with the experimental data over a wide range of temperatures. Our potential energy surface turns out to be substantially deeper than previous ab initio potentials. The minimum of −484 cm−1 has been found for the slipped parallel geometry at the intermolecular separation R = 3.54 Å and a saddle point at −412 cm−1 for the T-shaped configuration and R = 4.14 Å. Three minima and two first-order saddle points have been located on the pairwise-additive potential energy surface of the CO2 trimer. The nonplanar structure of C2 symmetry has been found to be 48.8 cm−1 more stable than the cyclic planar form of C3h symmetry, in disagreement with experimental observation. It is suggested that the relative stability of the two isomers cannot be reliably determined by pairwise-additive potential and inclusion of three-body forces is necessary for this purpose.
