  The cooperative effect plays a significant role in understanding the
intermolecular donor-acceptor interactions of hydrogen bonds (H-bonds,
D-H...A). Herein, using the benchmark method of high-precision ab initio, the
well-known cooperative effect is reproduced in elementary H-bonded systems with
different D and A atoms. That is, with the decreasing of intermolecular
distance, the D-H bond length first increases and then decreases, while the
H...A bond length decreases. On the contrary, when D and A are the same, as the
intermolecular distance decreases, the D-H bond length decreases without
increasing, which is referred to as the uncooperative effect. Further analyses
conclude that compared to cooperative H-bonded systems, uncooperative systems
at their respective equilibrium position have a larger core-valence bifurcation
(CVB) index (>0.022) and lower binding energies (<0.25 eV), showing a clear
linear inverse relationship related to H-bond strength. Therefore, the
intermolecular non-H-bonding interactions are predicted to reflect the
uncooperative characteristics, which is confirmed by high-precision ab initio
calculations. These findings provide a direction for the comprehensive
understanding of H-bonds.
