Computational study of rovibrational spectra of Van der Waals dimers and their isotopologues
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A new intermolecular potential energy surface, rovibrational transition frequencies, and line strengths are computed for OCS-OCS and CO2-CS2. The potentials were made by fitting energies obtained from explicitly correlated coupled-cluster calculations and fit using an interpolating moving least squares method. Rovibrational transition frequencies are also calculated for four isotopologues of the N2O dimer using a previously presented potential energy surface. The rovibrational Schroedinger equation for all three dimers is solved with a symmetry-adapted Lanczos algorithm and an uncoupled product basis set. All four intermolecular coordinates are included in the calculation. On the OCS-OCS potential energy surface, a previously unknown, cross-shaped isomer is found along with polar and non-polar isomers. For CO2-CS2, the previously found cross-shaped minima is found along with a slipped-parallel configuration. The associated wavefunctions and energy levels for each of these isomers is presented. To identify states that have a permanent dipole, both calculations of line strengths and vibrational parent analysis is used. For non polar states of, OCS-OCS, and N2O-N2O isotopologues, and all CO2-CO2 states, only vibrational parent analysis was used. Calculated rotational constants differ from their experimental counterparts by less than 0.001 wavenumbers for OCS-OCS and CO2-CS2, and less than 0.002 wavenumbers for any N2O-N2O isotopologue.