I think you could find this piece of documentary useful - https://manual.gromacs.org/current/reference-manual/functions/bonded-interactions.html

In GROMACS, hydrogen bonding is determined by a geometrical criterion, which considers specific angle and distance criteria between donor, hydrogen, and acceptor atoms.

For more details, you can refer to this link: https://manual.gromacs.org/documentation/current/reference-manual/analysis/hydrogen-bonds.html

"... Is it primarily electrostatic , or does it calculate the number of H-bonds based on the angle and distance criteria?..."

There is no contradiction between these two. Indeed, the nature of the H-Bonds in GROMACS is mainly electrostatic, the H-Bond is so to say “caused” by an interplay of electrostatic interactions between donor, acceptor and H atoms but also donor-acceptor LJ interactions. On the other hand, the algorithm “recognizes” the existence of a H-Bond applying geometric criteria (angles and distances).

I hope this helps

Paulo Netz

@PauloNetz

Thank you dear paulo

Thank you @Anand and @Alisa