The nature of electronic excitations in singly bonded C-60 dimer

Studying excited states of molecules is rather demanding computationally, a nd available theoretical methods usually lack a transparent interpretation scheme. Interpretation of electronic spectra is especially important if two or more fragments can be identified in the system. We developed a method to characterize different excited states in dimer mol ecules. Localized dimer wave functions are determined in the following mann er. Isolated monomer wave functions are projected into the exact subspace o f dimer orbitals leading to nonorthogonal localized MOs. Orthogonal localiz ed MOs emerge from these by Lowdin's procedure. The resulting orbitals have the characteristic feature that they are localized either on monomer "A", or monomer "B", but they preserve the local intramolecular symmetries withi n both fragments. In terms of these orbitals an all-single CI calculation i s executed. inspecting the CI eigenvectors, the excitations can easily be c lassified as to their localized vs charge transfer characters (A --> A, B - -> B, A --> B, and B --> A). This procedure was used in studying excited st ates of the singly bonded covalent buckminsterfullerene dimer. (C) 2000 Pub lished by Elsevier Science B.V. All rights reserved.