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I have constructed a solid-liquid interface model with water as the liquid phase. During the AIMD simulation, I used the NPT ensemble. However, I noticed that under the NPT ensemble, the entire model's lattice changes significantly, especially the volume of the water in the upper liquid phase, which leads to an inconsistent density. I would like to know if there is a way to maintain a constant density of 1 g/cm3 for the water box while using the NPT ensemble."

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In an AIMD simulation the system size will be very small, so a constant pressure ensemble will entail relatively large volume fluctuations. You should stick to a constant volume simulation unless you have a very compelling physical reason for modelling the isobaric volume changes your box would experience (and can run your AIMD simulation long enough to average over box size fluctuations).

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    $\begingroup$ Very thanks to your reply! This help me figure out why so many research papers conduct AIMD simulation with NVT ensemble. $\endgroup$ Commented Jun 11, 2023 at 3:04

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