A temperature trace that moves up and down can make it hard to decide whether an equilibration run is working. Should you stop when the curve first reaches its target, or keep running until it becomes perfectly flat? In an NVT simulation, neither a single reading nor a perfectly horizontal line is the goal. The useful question is whether the temperature’s running average has settled around the target, with ordinary fluctuations around it.
NVT keeps the number of particles, volume, and temperature constant. It is commonly the first equilibration phase after energy minimization: the system is brought toward its target temperature before a later NPT phase is used to stabilize density. Treating NVT as a checkpoint—not just a button to press—helps avoid moving on before the system is ready.
Start from a sensible input
Begin with the minimized structure and confirm that the minimization completed without unresolved problems and that the system looks reasonable. In SAMSON’s GROMACS Wizard, open the Equilibrate (NVT) tab. The input can be a GRO file from minimization or a previous equilibration run, or a suitable batch project. If you are continuing from the preceding step, the auto-fill button can populate the input path from the previous successful run.
Use the temperature plot as a decision aid
Default NVT parameters are provided for typical runs, and the Parameters section exposes commonly adjusted settings such as the time step and number of steps. The required duration depends on the system. The tutorial notes that 50–100 ps is typically sufficient, but this is a guide, not a pass/fail rule: if the temperature has not reached or stabilized near the target, more equilibration may be needed.
After the job finishes and results are loaded, inspect the temperature plot. In the tutorial example, the temperature stabilizes around the default target of 300 K. Your key checks are practical: is the running behavior around the intended value, and does it look stable enough for the next stage? Small fluctuations are normal. If the temperature is still settling, use the current NVT results as the input for another NVT run rather than treating the first run as automatically complete.
Keep the controls consistent
Temperature coupling settings include the thermostat, the groups being coupled, the time constant, and the reference temperature. For many projects, the documented starting point is velocity rescaling with a stochastic term (v-rescale) and a time constant of about 1 ps. The temperature specified for velocity generation should correspond to the coupling temperature.
If you couple separate groups—for example, Protein and non-Protein—make sure the number of time constants and reference temperatures matches the number of groups. Position-restraint options are filled automatically based on the input system. For a tutorial run, restore the default parameters; for a project with specific requirements, review custom settings carefully rather than changing several controls without a reason.
A simple NVT handoff checklist
- Start from the minimized structure or the results of the previous NVT run.
- Check that the target and coupling settings are appropriate and consistent.
- Run NVT locally, generate inputs for another environment, or use a cloud job when appropriate.
- Review the temperature plot after the results load; do not mistake normal fluctuations for failure.
- If the temperature has not stabilized, continue NVT from the latest results.
- Proceed to NPT when the temperature is stable enough for your workflow.
This approach turns the plot into a useful checkpoint: the aim is not to make every fluctuation disappear, but to judge whether the system is ready for the next equilibration phase. For the complete workflow and interface details, see the NVT Equilibration documentation.
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