Performance of the AM1, PM3, and SCC-DFTB methods in the study of conjugated Schiff base molecules

In this work, we study the performance of approximate quantum mechanical methods AM1, PM3 and SCC-DFTB in their description of conjugated Schiff base (SBn) models. The structure, isomerization barriers and proton affinities have been calculated in several different models of the retinal SBn, including different lengths of the polyene chain and different alkyl substitutions. Furthermore, the external effects originating from the protein environment of bacteriorhodopsin (bR) have also been investigated. Comparison of the results with those obtained from DFT calculations clearly indicates that the SCC-DFTB method can be considered as a promising method for the examination of these molecules and for explicitly including large parts of the protein environment in the calculations. While structures are well described by PM3, it largely underestimates rotational barriers. (C) 2002 Published by Elsevier Science B.V.