A pressure plot that swings dramatically can make it hard to tell whether an NPT equilibration is working. The key is not to treat every pressure fluctuation as a verdict on the simulation. In this step, the practical question is whether the system’s density has settled into a stable regime—and whether the results are steady enough to continue toward production molecular dynamics.
NPT equilibration keeps the number of particles, pressure, and temperature constant while the system adjusts. It follows energy minimization and NVT equilibration: after minimizing the structure and stabilizing its temperature, the next goal is to stabilize density. Pressure may fluctuate widely during equilibration, so a noisy pressure trace alone does not necessarily mean that the run has failed.
Read the density trend, not one moment
Look at how density changes over simulation time. A useful sign of progress is that its running average approaches a plateau rather than continuing to drift. Individual values can still fluctuate; what matters is whether the overall trend looks stable enough for your system.
In the tutorial’s example, the density stabilizes around 1030 kg/m3. That is close to the cited experimental value of 1000 kg/m3, while the expected density for the SPC/E model is about 1008 kg/m3. These values are context for that example—not a universal target for every system. Judge your result against the intended system and model.
A practical NPT workflow
- Start from the previous step. Open the Equilibrate (NPT) tab and provide the GRO file or batch project from minimization or NVT equilibration. If you are continuing from a previous successful run, the auto-fill button can populate the input path.
- Keep temperature coupling consistent. Reuse the temperature-coupling choices from NVT unless you have a clear reason to change them. If velocity generation is enabled, make sure its temperature is consistent too.
- Choose a reasonable initial duration. The documentation suggests that 100 ps is typically a good starting point, but the time required depends on system size and contents. Treat it as a starting point, not a guarantee that equilibration is complete.
- Inspect density over time. After the job finishes, review the plots in the NPT tab. The plots are generated and saved when the job finishes and the results are loaded. You can save a plot using the figure’s Save button.
- Continue if density is still drifting. If it has not stabilized, run NPT equilibration again using the results from the previous NPT step as the input. This lets equilibration continue from the current state rather than restarting from an earlier one.
Parameters worth checking
The standard Parameters section contains commonly adjusted settings, such as the integration time step and number of steps. The All… button opens additional GROMACS parameters. In the Pressure coupling section, the barostat, time constant, and reference pressure can be adjusted. The tutorial describes exponential relaxation pressure coupling with a stochastic term (c-rescale) and a time constant of about 5 ps as a typical choice for many projects—not as a rule for every system. Position-restraint options are filled automatically based on the input system.
When you need to adjust advanced settings, change parameters deliberately and keep track of what you changed. For a tutorial run, the instructions recommend resetting parameters to their defaults. SAMSON also lets you reset, load parameters from an MDP file, or save them to one; modified parameters are saved when SAMSON is closed.
Decide whether to proceed
Before moving to production, ask: Has density reached the desired regime? Does its trend look stable enough to continue? If not, extend NPT equilibration from the latest NPT results and check again. A stable density is the handoff signal; a wildly fluctuating pressure value by itself is not the whole story.
For the full workflow and interface details, see the GROMACS Wizard NPT Equilibration tutorial.
Note: SAMSON and all SAMSON Extensions are free for non-commercial use. Get SAMSON.