To study the inter-ring s/(p/p) covalent interactions between non-radical p-systems, five
structures of cyclodimers of benzene (C6H6)2 with all the real frequencies, i.e. o–p0-dibenzene (A),
the pentacyclic dimer (B), p–p0-dibenzene (C), syn–o,o0-dibenzene (D), and hexaprismane (E),
are obtained at the MP2/6-311G(d,p) level.