Specific features of extra strong intermolecular hydrogen bonds (H-bonds) are the electron density varying from 0.12 to 0.19 a.u. and the negative Laplacian values at the bond critical point. The Kirzhnitz approximation, widely used for estimation of the kinetic energy density from the crystalline electron density, leads to rather large errors (similar to 15%) for these bonds. The border between the extra strong and strong H-bonds locates at the O center dot center dot center dot O distance around 2.45 angstrom for crystals with the quasi-linear O-H center dot center dot center dot O moiety. The simple dependence between the H-bond energy and the O center dot center dot center dot O distance is suggested. (C) 2015 Elsevier B.V. All rights reserved.