Dear All,
I am trying to simulate a box of water having density 1gm/cm3 and 10,000 molecules using LAMMPS. The issue which I am facing is that I am not able to maintain the pressure constant. It keeps on fluctuating in the range of +-500 atm.
But still I plotted the mean square displacement(msd) and radial distribution function (rdf). The self diffusion coefficient which I calculated from msd was almost equal (1 % error) to the actual value in journal papers obtained for that water model.
Also the rdf plot exactly matched the plot given in journal papers. So, can I conclude that pressure fluctuations of such amplitude can be ignored and I should proceed further in MD?
Dear
it is necessary analyze the barostat that is using ( berendsen), ensemble systems, simulation time and descorrelacion process.
Regards
large fluctuations of instantaneous pressure are normal in MD, there is no discussion about this in LAMMPS documentation but you can read about it in Gromacs manual
http://www.gromacs.org/Documentation/Terminology/Pressure
"Whether or not pressure coupling is used within a simulation, the pressure value for the simulation box will oscillate significantly. Instantaneous pressure is meaningless, and not well-defined. "
and also in AMBER webpage http://ambermd.org/Questions/pressure.html
"pressure fluctuations are large because the pressure-volume isotherm of a liquid is extremely steep"
Using LAMMPS you can calculate and print average pressure over window of some time, fluctuations of average pressure should be much smaller
variable P equal press
fix PressAve all ave/time 10 1000 10000 v_P
thermo_style custom step temp f_TempAve press f_PressAve f_PEAve_Mol f_DensAve
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