yields a good agreement between bond valence sum and the valence of hydrogen (1). Fitted constants for R0 and b are R = 0.914 Å and b = 0.404 Å [Alig, Lösel, Trömel, 1994].
This constants are best on the fitting of 93 high quality neutron diffraction studies and yield for over 200 structures investigated (organic and inorganic) an agreement between calculated and observed valence with a standard derivations below 2% which is an error below 1 pm in the primary bond length.
Quite similar parameter have been found by Steiner and Saenger  and for an empirical curve see Brown .
A table of some bond valences using the above function is given below:
|R in Å||s in vu|
Even for long distances the bond valence for a hydrogen bond doesn't fall to zero and several of this bonds may lead to a stable structure as desribed in brucite. Still it seems justified for practical reasons to ignore everything above a certain limit, as the contributions are quite small.