CO Dimer: New Potential Energy Surface and Rovibrational Calculations Richard Dawes,* ,† Xiao-Gang Wang,* ,‡ and Tucker Carrington, Jr.* ,‡ † Department of Chemistry, Missouri University of Science and Technology, Rolla, Missouri 65409, United States ‡ Chemistry Department, Queen’s University, Kingston, Ontario K7L 3N6, Canada * S Supporting Information ABSTRACT: The spectrum of CO dimer was investigated by solving the rovibrational Schrö dinger equation on a new potential energy surface constructed from coupled-cluster ab initio points. The Schrö dinger equation was solved with a Lanczos algorithm. Several 4D (rigid monomer) global ab initio potential energy surfaces (PESs) were made using a previously reported interpolating moving least- squares (IMLS) fitting procedure specialized to describe the interaction of two linear fragments. The potential has two nonpolar minima giving rise to a complicated set of energy level stacks, which are very sensitive to the shapes and relative depths of the two wells. Although the CO dimer has defied previous attempts at an accurate purely ab initio description our best surface yields results in good agreement with experiment. Root-mean-square (rms) fitting errors of less than 0.1 cm -1 were obtained for each of the fits using 2226 ab initio data at different levels. This allowed direct assessment of the quality of various levels of ab initio theory for prediction of spectra. Our tests indicate that standard CCSD(T) is slow to converge the interaction energy even when sextuple zeta bases as large as ACV6Z are used. The explicitly correlated CCSD(T)-F12b method was found to recover significantly more correlation energy (from singles and doubles) at the CBS limit. Correlation of the core-electrons was found to be important for this system. The best PES was obtained by extrapolation of calculations at the CCSD(T)(AE)- F12b/CVnZ-F12 (n = 3,4) levels. The calculated energy levels were compared to 105 J ≤ 10 levels from experiment. The rms error for 68 levels with J ≤ 6 is only 0.29 cm -1 . The calculated energy levels were assigned stack labels using several tools. New stacks were found. One of them, stack y 1 , has an energy lower than many previously known stacks and may be observable. I. INTRODUCTION The CO dimer has intrigued spectroscopists for many years. It has long been understood that the rovibrational levels can be organized into stacks associated with a given symmetry and a value of K, the quantum number for the projection of the angular momentum along the axis joining the two CO monomers. Some of the lowest stacks can be unambiguously associated with one of the two shallow wells on the PES (potential energy surface). At higher energies, the wave functions delocalize, and the stacks cannot be separated into two clearly distinguishable groups. 1-7 To compute the rovibrational spectrum of CO dimer, one assumes that it is sufficient because the CO stretch frequencies are much higher than the intermolecular frequencies, to solve a Schrö dinger equation with four vibrational coordinates specify- ing the shape of the dimer and three Euler angles specifying the orientation of the dimer. One way to solve this Schrö dinger equation is to expand the potential in terms of coupled angular functions so that all matrix elements of the potential can be determined analytically. Several calculations have been done with this approach. 8 An alternative is to use quadrature. This has the advantage of obviating the expansion and its associated error. Quadrature can be used with the Lanczos algorithm to calculate a spectrum without computing individual potential matrix elements. It is only necessary to evaluate matrix-vector products, and this can be done by evaluating sums sequentially. 9-18 The CO dimer is weakly bound, and it is therefore necessary to use basis functions that span the entire configuration space of the molecule. This is easily done if one use the polar-type coordinates commonly referred to as polyspherical coordinates. 19 Owing to the fact that it is not necessary to store the Hamiltonian matrix, it is straightforward to increase the number of basis functions and the number of quadrature points to ensure that the energy levels are converged. The accuracy of spectrum obtained from the Lanczos calculation is limited only by the quality of the PES. To make a good PES for CO dimer, one needs both a very accurate ab initio method and an excellent fitting procedure. The spectrum is very sensitive to the relative depths and the shapes of the two wells. There was some debate in the literature between 1999 and 2001 regarding the accuracy of CCSD(T) for this system. 20-22 A SAPT based diagrammatic analysis of Special Issue: Joel M. Bowman Festschrift Received: May 18, 2013 Revised: June 4, 2013 Published: June 5, 2013 Article pubs.acs.org/JPCA © 2013 American Chemical Society 7612 dx.doi.org/10.1021/jp404888d | J. Phys. Chem. A 2013, 117, 7612-7630