Adjusted connection atoms for combined quantum mechanical and molecular mechanical methods

Connection atoms are proposed as an alternative to link atoms in semiempirical hybrid calculations that divide a system at a C-C single bond into a quantum 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 definitions of these interactions are given for three QM/MM coupling models (A mechanical embedding, B/C electronic embedding without/with MM polarization). Semiempirical connection atom parameters are derived for three standard methods (MNDO, AM1, PM3) such that the adjusted connection atoms closely reproduce the geometrical and electronic properties of methyl groups. The corresponding deviations are generally smaller than the intrinsic errors of these methods. QM/MM test calculations on proton affinities confirm the usefulness of the adjusted connection atoms, particularly in coupling model B. Connection atoms are conceptually superior to link atoms in that they do not introduce extra centers and thus lead to well-defined potential surfaces. In addition, they allow an improved semiempirical description of the QM/MM interactions.