PROTON-TRANSFER IN THE GROUND AND LOWEST EXCITED-STATES OF MALONALDEHYDE - A COMPARATIVE DENSITY-FUNCTIONAL AND POST-HARTREE-FOCK STUDY

Intramolecular proton transfer in the ground and the lowest two excite d electronic states of malonaldehyde has been investigated by using de nsity functional and post-Hartree-Fock methods. Our best estimates of the energy barriers governing proton transfer in the ground and lowest triplet state are quite low (4.3 and 6.6 kcal/mol, respectively), whe reas a significantly higher barrier (12.0 kcal/mol) is obtained for th e second triplet state. The coupled cluster approach provides reliable results already with relatively compact basis sets, its only drawback being the very unfavorable scaling with the number of active electron s. Among the cheaper methods, those based on the many-body perturbativ e approach provide good results for the ground electronic state, but t heir performances strongly deteriorate for excited states. The overest imation of correlation energy by conventional density functional metho ds produces an excessive degree of conjugation in the backbone of malo naldehyde with the consequent underestimation of energy barriers gover ning proton transfer. A more coherent picture is offered by a hybrid d ensity functional/Hartree-Fock approach, which couples good structural predictions with a reduced, although still not negligible, underestim ation of energy barriers. Furthermore, different electronic states are described with comparable accuracy. (C) 1996 American Institute of Ph ysics.