Becke weight function for real space partitioning was used to constrain electronic charges on each fragment. To avoid the poor scaling of the Becke method with the system size, the element-specific cutoff radii of 3.2, 2.5, 2.5, and 2.5 Å have been applied to C, N, H, and O atoms, respectively. The Becke cell boundaries were shifted using ionic radii for the atoms: 46 0.76, 0.71, 0.31, and 0.66 Å for C, N, H, and O atoms, respectively. The constrained charge convergence criterion was set to 10 -2 e. The systems were centered in cubic boxes with a 25 Å cell size in the z-axis direction.