Combining ab initio and density functional theoreies with semiempirical methods

摘要

For large reactive systems, the calculations of energies can be simplified by treating the active part with a high-level quantum mechanical (QM) (ab initio or density function) approach and the environment with a less sophisiticated semiempirical (SE) approach, as an improvement over the widely used hybrid quantum mechanical/molecular mechanical (QM/MM) methods. An example is the interaction between an active regin of an enzyme and its immediate environment. One such method is the original "Our-own-N-layer Integrated molecular Orbital + Molecular Mechanics (ONIOM)" approach. In this paper, the interaction between the QM and SE region is described explicity by two different schemes. In the iterative QM/SF schemes (QM/SE-I), the electrostatic interaction and polarization effects are introduced explicity for both the QM and SE atoms by a self-consisent procedure based on either polarizable point charges or the electron density. In the noniterative QM/SE scheme, based on the ONIOM model (QM/SE-O), the exchange (Pauli repulsion) and charge transfer effects are taken into account at the SE level, in addition to the explicit electrostatic interaction and polarization between the two regions. Test calculations are made on a number of model systems (including small polar or charged molecules interacting with water and proton transfer reactions in the presence of polar molecules or an extended hydrogen-bond network). The quantitative accuracy of the results depend on several parameters, such as the charge-scalingormalization facors for the SE charge and the QM/SE van der Waals parameters,which can be chosen to optimize the result. For the QM/SE-O approach, the reslts are more sensitive to the quality of the SE level (e.g., self-consistent-charge density-functional-tight-binding vs AM1) than the explicit interaction between QM and SE atoms.