Dear users:

I believe that the easiest way to calculate the number of water molecules in the first hydration shell of a macromolecule or molecular cluster is as follows.

## Set your cutoff as determined from g_rdf
DIST=0.435

## Calculate the data using g_mindist -respertime
echo -e "SOL_OW\nProtein_heavy\n" | g_mindist -s my.tpr -f my.xtc -n my.ndx -d ${DIST} -respertime -od od.xvg -or or.xvg

## Extract the desired numerical values from or.xvg
grep -v '[#|@|S]' or.xvg |awk '{s=0;for(i=2;i<=NF;i++)if($i<'${DIST}')s++;print $1,s}' > water_count.dat

This method does not work with gromacs 4.0.x. I am using g_mindist from version 4.5.3 here. From my tests, I get the correct values when using g_mindist version 4.5.3 and a .tpr and .xtc from gromacs version 4.0.7.

This is a final comment from the original post titled "interfacial water molecules"
http://lists.gromacs.org/pipermail/gmx-users/2010-December/056912.html

I don't believe that there is currently a flag to output this number directly from g_mindist.

Chris.

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