water flow in nanochannel

tnx steve,yes i apply the total force to the oxygens instead of mass center of water molecules,but it gives unexpectable conclude,in poiseuille flow the velocity profile must to be parabolic,but when apply force to oxygens the velocity profile becames very miningless and oscilating.but when i apply the force on oxygens and hydrogen,the velocity profile becames parabolic whit maximum value in center of channel but my maximum is 3 times smaller than simillar article,in your view what can i do for this probloem?thank you very much

I don’t know. I’ve told you my ideas. You can write your
own fix if you wish to apply force to the COM of each
water. If you are keeping the water rigid with fix shake
then it is a good idea to apply a force proportional
to the mass of the O and H atoms, so that the geometry of each
water molecule is not perturbed (which fix shake would
then try to correct for).