Charge Transfer Across ONIOM QM:QM Boundaries: The Impact of Model System Preparation.

The inability to describe charge redistribution from regions I to II at the high level of theory imposes limitations on the general applicability of the our own N-layered integrated molecular orbital and molecular mechanics (ONIOM) method. In this report, we exploit the most inexpensive components of an ONIOM QM:QM calculation to provide a new method which has the ability to describe such charge-transfer effects with only a nominal increase in computational effort. Central to this method is the model system preparation step, in which an one-electron potential is optimized to shift density into or out of a defined buffer region. In this initial effort, we treat the link atoms on the model subsystem as the electron buffer region and swell or diminish the link-atom nuclear charges to shift electron density into or out of the buffer region. Due to the relatively small computational cost of the model-low calculation, this procedure can be iteratively optimized to produce a charge distribution equal to the real-low calculation. Initial results for a test set of 20 reaction energies and 8 different combinations of high and low levels of theory show improvements of more than 35% over the standard ONIOM QM:QM approach, with improvements of up to 50% for some high and low combinations.