In this paper, we describe a novel procedure that fits the protein and reaction field potentials in the region of the QM system with artificial point charges. These charges are placed on a sphere surrounding the QM system plus a few charges inside the sphere. This procedure allows us to perform DFT calculations that involve an inhomogeneous dielectric environment in a fully self-consistent way. This improves the accuracy of calculations in comparison with previous work. The improved results show, however, that although the absolute energies change significantly, the energy differences between the protonated and deprotonated states of His291 remain close to the previously reported ones. Thus, the new improved pKa values remain close to the ones reported earlier providing further support for the proposed His291 model of the CcO pump.

Fitting of the external electrostatic field acting on the QM system with point charges. The solid line shows the actual potential on all atoms of the QM system given by MEAD (protonated oxidized state of the binuclear center, final iteration). Crosses show the results of its fitting by a set of point charges according to the proposed procedure (see original paper for all computational detail).