Adjusted connection atoms for combined quantum mechanical and molecular mechanical methods

Connection atoms are proposed as an alternative to link atoms in semiempiri cal hybrid calculations that divide a system at a C-C single bond into a qu antum mechanical (QM) and a molecular mechanical (MM) region. A connection atom interacts with the other QM atoms as a specially parametrized QM atom, and with the other MM atoms as a standard carbon MM atom. Detailed definit ions of these interactions are given for three QM/MM coupling models (A mec hanical embedding, B/C electronic embedding without/with MM polarization). Semiempirical connection atom parameters are derived for three standard met hods (MNDO, AM1, PM3) such that the adjusted connection atoms closely repro duce the geometrical and electronic properties of methyl groups. The corres ponding deviations are generally smaller than the intrinsic errors of these methods. QM/MM test calculations on proton affinities confirm the usefulne ss of the adjusted connection atoms, particularly in coupling model B. Conn ection atoms are conceptually superior to link atoms in that they do not in troduce extra centers and thus lead to well-defined potential surfaces. In addition, they allow an improved semiempirical description of the QM/MM int eractions.