This journal is © the Owner Societies 2019 Phys. Chem. Chem. Phys., 2019, 21, 15871--15878 | 15871 Cite this: Phys. Chem. Chem. Phys., 2019, 21, 15871 Explicitly correlated potential energy surface of the CO 2 –CO van der Waals dimer and applications† Ayda Badri, ab Leonid Shirkov, c Nejm-Eddine Jaidane a and Majdi Hochlaf * b The four-dimensional-potential energy surface (4D-PES) of the CO 2 –CO van der Waals complex is generated using the explicitly correlated coupled cluster with single, double, and perturbative triple excitation (CCSD(T)-F12) method in conjunction with the augmented correlation-consistent triple zeta (aug-cc-pVTZ) basis set. This 4D-PES is developed over the set of inter-molecular coordinates and where the CO 2 and CO monomers are treated as rigid rotors. Afterwards, analytic fits of this 4D-PES are carried out. In addition to the already known C-bound and O-bound stable structures of CO 2 –CO, we characterise a new isomer: it has a T-shaped structure where the O atom of the CO 2 moiety points into the centre of mass of CO. We also find the saddle points connecting these minimal structures. This new isomer may play a role during the intramolecular isomerization processes at low energies. Then, the 4D-PES expansion is incorporated into bound vibrational state computations of C-bound and O-bound complexes. We also computed the temperature dependence of the second virial coefficient for CO 2 –CO. A good agreement with experiments is found. I. Introduction The van der Waals complex between carbon dioxide and carbon monoxide, CO 2 –CO, is formed by two molecules of consider- able importance in atmospheric, planetary, environmental and industrial media. Thus, numerous theoretical and experimental studies have been devoted to this dimer. Back in 1989, the first detection and characterization of CO 2 –CO was performed by Legon and Suckley 1 using infrared and microwave spectrosco- pies. These authors identified a T-shaped cluster where the carbon atom of CO points to the carbon atom of CO 2 (denoted hereafter as ‘C-bound’). Three years later, Parish et al. 2 used the molecular mechanics for clusters (MMC) model. In addition to the C-bound complex, a second minimal structure where the oxygen atom of CO points into the carbon atom of CO 2 (denoted hereafter as ‘O-bound’) was identified. Then a series of experi- mental works followed, 3–8 which allowed the determination of the molecular geometrical parameters and the intermolecular vibrational frequencies of the C-bound complex either in the gas phase or trapped in cold rare gas matrices. It was only in 2015 that the O-bound complex was definitely observed in the gas phase using IR spectroscopy by Sheybani-Deloui et al. 9 confirming hence the earlier theoretical predictions and the tentative identifi- cation of this complex trapped in Ar matrices by Raducu et al. 6 This spectroscopic study at a high level of precision confirmed thus the existence and the stability of the O-bound structure. Theoretically, various ab initio methodologies were applied to CO–CO 2 since the work by Parish et al. These studies include density functional theory (DFT), 10 Moller–Plesset (MP2), 10,11 Sym- metry Adapted Perturbation Theory (SAPT) 8 and standard 12 and explicitly correlated 13 coupled cluster approaches. They showed that the C-bound dimer is more stable than the O-bound one. No other structures were found. To date, there is no multi dimensional potential energy surface (PES) for the CO 2 –CO complex. Indeed, previous studies are limited to geometrical optimizations followed by harmonic frequency computations. Note that Uteva et al. 12 generated an interpolated potential energy surface constructed over some specific geometrical configurations (including minimal structures) using Gaussian processes, where electronic compu- tations were carried out for 135 training points. Although this kind of PES allows accurate deduction of macroscopic properties (e.g. temperature dependence of second virial coefficient), this PES cannot be used for spectroscopic purposes such as the determination of the pattern of the rovibrational levels of a Laboratoire de Spectroscopie Atomique, Mole ´culaire et Applications LSAMA LR01ES09, Universite ´ de Tunis El Manar, Tunis, Tunisia b Universite ´ Paris-Est, Laboratoire Mode ´lisation et Simulation Multi Echelle, MSME UMR 8208 CNRS, 5 bd Descartes, 77454 Marne-la-Valle ´e, France. E-mail: hochlaf@univ-mlv.fr c Faculty of Chemistry, Adam Mickiewicz University, Umultowska 89b, 61-614 Poznan ´, Poland † Electronic supplementary information (ESI) available: Vibrational frequencies of the CO 2 –CO isomers. We give also the two programs used for the fits of the 4D potential. See DOI: 10.1039/c9cp02657f Received 10th May 2019, Accepted 26th June 2019 DOI: 10.1039/c9cp02657f rsc.li/pccp PCCP PAPER Published on 28 June 2019. Downloaded by KEAN UNIVERSITY on 7/17/2019 1:47:33 PM. View Article Online View Journal | View Issue