Electrostatic Potential-Based Method of Balancing Charge Transfer Across ONIOM QM:QM Boundaries.

Abstract

The inability to describe charge redistribution effects between different regions in a large molecule can be a source of error in an ONIOM hybrid calculation. We propose a new and an inexpensive method for describing such charge-transfer effects and for improving reaction energies obtained with the ONIOM method. Our method is based on matching the electrostatic potential (ESP) between the model system and the real system. The ESP difference arising due to charge redistribution is overcome by placing an optimum one electron potential at a defined buffer region. In our current implementation, the link atom nuclear charge is optimized iteratively to produce a model low ESP distribution equal to that in the real low calculation. These optimum charges are relatively small in magnitude and corroborate physical intuition. This new ESP-ONIOM-CT method is independent of any arbitrary definition of charges, is defined on the basis of a physical observable, and is less basis set dependent than previous approaches. The method is easily extended for studying reactions involving multiple link atoms. We present a thorough benchmark of this method on test sets consisting of one- and two-link atom reactions. Using reaction energies of four different test sets each with four different combinations of high:low levels of theory, the accuracy of ESP-ONIOM-CT improved by 40-60% over the ONIOM method.