Interactions between special pairs (SP) in the photosynthetic reaction center were studied using quantum chemical calculations in conjunction with the quantum theory of atoms in molecules. Two histidines, coordinated with Mg2+ ions of SP, weaken the interaction between SP halves. The acetyl and non-polar groups of SP stabilize SP in addition to the interaction between the macrocycles themselves. Correlations between the interaction energies and the sum of electron densities over the bond critical points of intermolecular interactions (interqb) were found, suggesting that using electron-density-based interaction energy is a useful alternative to using the interaction energy with a counterpoise corrected method. (C) 2012 Elsevier B. V. All rights reserved.