First, we briefly discuss the size-extensivity of single-state CASPT2 with and without IPEA shift using up to five ethylene molecules, each of which has been optimized at the Hartree-Fock/cc-pVDZ level of theory. For these calculations only, the imaginary level shift parameter was set to zero. Each ethylene molecule was separated by 100.0 Å, and the active space for each monomer consisted of two electrons in two (π and π * ) orbitals (CAS(2e,2o)