# simulating poiseuille water flow in nanochannel with copper walls in lammps

Hi dears,
I am simulating poiseuille water flow with 1058 water molecules(SPC/E model) in an nanochannel with copper(cu) walls with lammps ,with real units , the height of channel is 31.66 angstrome that is equal to 10 times bigger than water molecules diameter . but I have 2 problem in that,i thank you very much if you help me . the first is this that i fix the temperature of water molecules at 300 K in this way :
fix 1 all nve # all group means cu group(walls) and the water group (oxygen and hydrogen group)(flow)
fix 2 cu nvt temp 300.0 300.0 100.0
fix 3 water temp/rescale 1 300.0 300.0 0.001 1.0
program is run in 200000 time steps,the temperature raises from 1K in first timestep and in timestes almost 20 in remained on 219.8891K and doesnot chane after that and stayes fixed on 219.8891K till program be finished(timestep 200000)! But if i fix the temperature on 410K : (fix 3 water temp/rescale 1 410.0 410.0 0.001 1.0) then so the temperature will be stayed on 300K till last time step,with no change,meaning the favorite temperature,but I donot know why, i want to fix on 300K and get the 300K . I donot know why this happens,please if you know its cause or you can give me a propose please tell me . thank you so much
The another prob is this that the reference article applied unitless force(lj units)=2.5 to the mass center of each water molecules and , 2.5 unitless for force is equal to 0.123 kcal/mol in SPCE water model.so I apply 0.123/3 = 0.041 kcal/mol to each atom in water molecules,that cause to apply 0.123kcal/mol to each mass center of water molecules . but the maximum velocity that I reach (in center of cannel)is 0.0049 m/s = 490m/s while the reference article tells maximum velocity is 8 (unitless) , is equal to 1490 m/s . in your view this my way for applying the force differs with reference? if yes so why? And whether if it differs,can it effect this much on velocity?(490 to 1490) . please if you can answer me . thanks a lot .

A couple issues with your commands. You are applying two integrators
to Cu (NVE and NVT). This is bad. You should have got a warning
that you were doing this. Your water is flowing, so you need to think
about how you measure the temperature. E.g. compute temp/profile
is probably needed. You should look at all the compute temp variants
and figure out how to monitor the thermal temperature, and thermostat
it is needed.

Steve