toma0052 wrote:
Hello,
     I am wondering about a response I got a while ago about indexing
atoms.  I am not very familiar with awk, but it seems like a fairly simple
way to search through a .gro file and find the solvent molecules with a
particular z-coordinate range, as I need for my simulation.  My question
however, concerns  implementing this into an md run.  I would like to add
an acceleration to solvent molecules that are within a certain distance
from a lipid bilayer.  However, if the solvent molecules diffuse too far
away during the md run, I no longer want them to have an acceleration. Therefore, it appears that at every time step I need to perform a check to see which solvent molecules are close enough to warrant an acceleration. So, is there a way that I can incorporate the awk file such that it runs
and creates a particular index group at every time step?  If not, is there
some other way that I can check at each time step (or every several time
steps) which solvent molecules have a particular z-coordinate and give them
an acceleration?

I presume you've read the relevant manual sections. If there's nothing useful there, the only way to do something like this would be to get inside the C code, loop over all solvent molecules after do_force to check for their position, and then adjust the force on them accordingly.

Mark
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