Streamlined small part of pdb2gmx
In the absence of manual assignment of histidine protonation, the
automated assignment mechanism analysed the existence of hydrogen
bonds before determining whether any histidine residues existed in
the current chain. This led to confusing output on (for example)
DNA chains.
This improvement checks for the existence of at least one histidine
before analysing and reporting on the hydrogen-bonding network.
Thus pdb2gmx will process chains lacking histidine slightly faster,
and be negligibly slower for those including a histidine.
It's also a little more verbose about why it is searching for
hydrogen-bonding networks.