FMO interactions in intermolecular complexes

H-bonding, the prevalent supramolecular force The donor-acceptor FMO interactions are also involved 

The covalent character in H-bonding spans a continuum from the essentially electrostatic interactions to the strong covalency in 3-center, 4-electron systems (F-H - F ). The significance of stereoelectronics parallels the increase in covalent characters. Stereoelectronics are especially important in the strong, short low-barrier H-bonds often encountered in enzyme catalysis. On the other hand, the bifurcated and trifurcated bonds, where the importance of individual orbital contacts is diluted, often sacrifice stereoelectronics to reach the maximum Coulombic stabilization. 

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