regarding fix NPT damping parameter

Hi all,

I am using fix NPT for thermostating with temp. and pressure damping parameter as 100 and 1000 respectively. These are timesteps for which i want to damp these parameters as manual mentions them appropriate values .

Therefore my 1st question is should it be placed as it is like 100 and 1000 or

as 0.1 (1000.001), and 1 (10000.001))

timestep 0.001 ### 1 fs
velocity all create 300 2223333 temp new
fix 1 all npt temp 300 300 100 iso 0.0 0.0 1000
### fix 1 all npt temp 300 300 0.1 iso 0.0 0.0 1
run 1000000

#### run 1000

My second query is if the pressure damping parameter is 1000, then no. of steps in the run command should be greater than 10 00 000 (because 1000000*0.001 = 1000). Is it?

The Documentation says “A good choice for many models is a Tdamp of around 100 timesteps. Note that this is NOT the same as 100 time units for most units settings”

Now Tdamp being in time units, in your case, it should be 0.1 (1000.001) and Pdamp should be 1 (10000.001).


The tdamp and pdamp parameters have units of time, not timestep. It is a good heuristic to set them to 100dt and 1000dt, respectively, where dt is the timestep size, also in units of time. In your example, that means 0.1 and 1. In response to your second question, if you are using a thermostat and barostat to generate a representative sample the NPT ensemble distribution, it is necessary to that the simulation cover at least a few hundred multiples of tdamp and pdamp, since these are roughly the characteristic timescales on which energy and volume are exchanged with the reservoirs. Anything less will not gives you a representative sample. Note that I said necessary, not sufficient. Other processes in the simulation may evolve even more slowly than tdamp and pdamp, and will necessitate running the simulation long enough to capture those.

Thank you Swastik and Aidan .