Comparison of methods for calculating the properties of intramolecular hydrogen bonds. Excited state proton transfer

A series of molecules related to malonaldehyde, containing an intramolecula r H-bond, are used as the testbed for a variety of levels of ab initio calc ulation. Of particular interest are the excitation energies of the first se t of valence excited states, n pi* and pi pi*, both singlet and triplet, as well as the energetics of proton transfer in each state. Taking coupled cl uster results as a point of reference, configuration interaction-singles-se cond-order Moller-Plesset (CIS-MP2) excitation energies are too large, as a re CIS to a lesser extent, although these approaches successfully reproduce the order of the various states. The same may be said of complete active s pace self-consistent-field (CASSCF), which is surprisingly sensitive to the particular choice of orbitals included in the active space. Complete activ e space-second-order perturbation theory (CASPT2) excitation energies are r ather close to coupled cluster singles and doubles (CCSD), as are density f unctional theory (DFT) values. CASSCF proton transfer barriers are large ov erestimates; the same is true of CIS to a lesser extent. MP2, CASPT2, and D FT barriers are closer to coupled cluster results, although yielding slight underestimates. (C) 1999 American Institute of Physics. [S0021-9606(99)302 26-9].