Hi.
I want a system to run under certain temperature (50~70 Kelvin). I used following commands for velocity and temperature:
velocity hydrogen create 70 7867431 rot yes dist gaussian
fix 1 all nvt temp 70 70 100 tchain 1
thermo_style custom step pe lx ly lz press c_eatoms temp v_eb
After running the simulation I can see the temperature varies wildly between 7~260 Kelvin which is not expected. How can I get my desired temperature (50~70 Kelvin)?
How large/small is your system (i.e. how many atoms)?
How long is your simulation (i.e. how many steps)?
What is your units setting?
Does the temperature fluctuate only at the beginning of the simulation or do you have the same kind of fluctuations over the whole trajectory?
There are total 72 atoms.
5,000 steps.
Units: Real
I am getting same kind of fluctuations through the whole simulation. Even if I increase the steps to 10,000 the fluctuations are still there.
72 atoms is very small. The smaller the system, the larger the fluctuations. Thermodynamically speaking, temperature is only well defined in the limit of an infinitely large system.
But before running with a thermostat, you should do a simulation with fix nve and observe whether you can conserve energy. If not, you may have chosen too large a time step.
Please also note that you are assigning an initial temperature to a group, but then apply the thermostat of fix nvt to the entire system.