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 init
io 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 d
imer, while two energetically very similar stable minima, exhibiting n
ear T-shaped C(s) and slipped-parallel C2h spatial arrangements, were
predicted for the (PCCP)2 dimer. Our predictions for the (NCCN)2 compl
ex are in agreement with a recent gas phase spectroscopic study. No ex
perimental 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 inc
luding electron correlation effects and higher polarized/diffuse funct
ions are discussed.