Calculations presented in this work were carried out for hinge-like molecular models composed of the inner (B)Chl a molecule and the axial His in vacuum and in protein binding pockets. We denote these complexes as [Cof• • • His], where Cof is the P D1 , P A or D A co-factor from RCs of PSII, PSI or purple bacteria, respectively. Subscripts D1 and A are used here to label the protein sub-units to which the co-factors belong. We consider models with i) protonated and deprotonated π-N position of His and ii) the long hydrocarbon tails R of (B)Chl a truncated and subsequently replaced by -CH 3 or left unchanged (for notations, see Fig. ). In order to distinguish these different complexes, an additional label "t" is introduced. Thus, the hinge model composed of the truncated P D1 and the protonated His is denoted as [tP D1 • • • HisH], whereas its deprotonated and untruncated counterpart is written as [P D1 • • • His] -. In addition to these structural changes, molecular complexes with modified relative orientations of (B)Chl a and His were generated in the vacuum. For the intermolecular distances between the magnesium ion and τ -N of His (see Fig. ) coordinated to it, values of ∆R = 0.00, ±0.01 and ±0.03 Å relative to the distances R characteristic to the crystal structure were considered. The angle θ between the τ -N atom of His, magnesium, and the middle of the (B)Chl a C-17-C-18 bond (see Fig. ) was modified by ∆θ = 0, ±5, ±10, and ±15 degrees.