The molecular structure of the (NCCN)2and (PCCP)2van der Waals dimers

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Abstract
The stationary points on the potential energy surfaces of the (NCCN)2 and (PCCP)2 van der Waals homodimers have been examined at the ab initio Hartree-Fock level of theory employing a 4-31G basis set. Only a T-shaped C2v stable minimum energy structure was found for the (NCCN)2 dimer, while two energetically very similar stable minima, exhibiting near T-shaped Cs and slipped-parallel C2h spatial arrangements, were predicted for the (PCCP)2 dimer. Our predictions for the (NCCN)2 complex are in agreement with a recent gas phase spectroscopic study. No experimental gas phase study seems to have been reported for the (PCCP)2 dimer, and our results indicate that an equilibrium between a near T-shaped and a parallel form is likely to occur. The implications of including electron correlation effects and higher polarized/diffuse functions are discussed.