A molecular dynamics system can reach its target temperature and still be a long way from being ready for production. After NVT equilibration, the next question is whether the system’s density has settled. That is the purpose of NPT equilibration: to let pressure and volume adjust while keeping the number of particles, pressure, and temperature controlled.
This step can be confusing because pressure often fluctuates substantially. A noisy pressure trace does not, by itself, tell you whether equilibration has failed. Density is a more useful quantity to follow for this practical checkpoint: look for its running behavior to settle into a stable regime before moving on.
Start from the latest equilibrated structure
In SAMSON’s GROMACS Wizard, open the Equilibrate (NPT) tab. Use the GRO file or batch project produced by the previous minimization or equilibration step. If you are continuing directly from NVT, the auto-fill button can populate the input path from the previous successful run; you can also select a GRO file yourself.
Keep the temperature setup consistent
A useful safeguard is to carry forward the temperature-coupling choices used during NVT equilibration. The NPT step should not accidentally introduce a different temperature setup: check that the temperature-coupling parameters match those from NVT, and check the temperature used for velocity generation if that option is enabled. Position-restraint options are filled automatically based on the input system.
The Wizard provides typical default parameters, including commonly adjusted settings such as the integration time step and number of steps. For pressure coupling, the tutorial describes exponential-relaxation coupling with a stochastic term (c-rescale) and a time constant of about 5 ps as a choice suitable for most projects. These are starting points, not a substitute for checking whether a particular system has equilibrated.
Give density time to settle
About 100 ps is presented as a reasonable starting timeframe, but the time required depends on the size and contents of the system. Judge the result from the density plot rather than treating a preset duration as proof of equilibration. If the density is still changing rather than approaching a stable regime, continue NPT equilibration from the results of the previous NPT run. In other words, an unsettled result is a cue to continue—not a reason to discard the work already done.
Pressure may fluctuate widely during equilibration and simulation. Avoid expecting a perfectly flat pressure trace. Instead, use the plots together: inspect pressure and density over time, and pay particular attention to whether density has become stable enough for your system. The tutorial’s example reaches a density of 1030 kg/m3, close to the experimental value of 1000 kg/m3; it also notes an expected value of about 1008 kg/m3 for the SPC/E model. Those figures illustrate the example, not a universal target for every molecular system.
Run, inspect, and continue if needed
You can generate input files for another environment, equilibrate locally, or launch the computation in the cloud. For a local run, monitor progress in the Output window or use Local jobs to check job status. When the job finishes, choose whether to import the full trajectory, only the last frame, or a range of frames. The results and automatically generated plots are saved in the results folder; NPT runs use the suffix _npt.
Before moving to production MD, ask one practical question: has density reached a stable regime that is appropriate for this system? If not, run another NPT equilibration using the current NPT results as input. Once it has, continue to Step 5: Production Molecular Dynamics Simulation.
For the complete walkthrough and parameter details, see the GROMACS Wizard NPT Equilibration tutorial.
Note: SAMSON and all SAMSON Extensions are free for non-commercial use. Get SAMSON at www.samson-connect.net.