The QM/MM approach is particularly useful in the study of heterogeneous catalysis. ChemShell supports embedded cluster calculations [2,4] in which the bulk material or surface is represented by a finite MM cluster model, optionally with additional point charges to mimic the effect of bulk electrostatics. The active site is modelled by a high-level QM calculation with electrostatic embedding, where the cluster environment is represented by point charges in the QM Hamiltonian. This setup is intended to give an optimal balance between accuracy at the active site and computational expense.
ChemShell may be built as a serial or parallel program, although the current release (v3.3) does not contain any parallel algorithms of its own. The parallel version instead is intended to exploit any parallelism contained in the packages used to evaluate the energy and gradient. Usually the evaluation step of the calculation is by far the most computationally expensive, and so parallelisation of this step should scale well. However, on large supercomputers such as HECToR, where calculations involving thousands of cores are routine, the parallel algorithms in the external programs often do not continue to scale efficiently. In this domain we expect there to be significant gains from adding a second layer of parallelism at the ChemShell level.
Tom Keal 2010-06-29