Link molecule method for quantum mechanical/molecular mechanical hybrid simulations

We present a new coupling method for hybrid simulations in which the system is partitioned into covalently linked quantum mechanical (QM) and molecular mechanical (MM) regions. Our method, called the “link molecule method (LMM),” is substantially different from the link atom methods in that LMM is free from the delicate issue of how to remove the additional degrees of freedom with respect to the position of the virtual atoms linking the QM and the MM regions. The force acting on the atom at the regional boundary is obtained in a simple form based on the total energy conservation. The accuracy of LMM is demonstrated in detail using a system of silicon partitioned into the QM and the MM region at the (1 0 0) boundary plane. This condition has been difficult to simulate by conventional methods employing the link atoms because of the strong repulsion between the nearby link atoms.