Question about the structure in the final dynamical matrix of NPT run

[PutuB]

Dear SSCHA developers,

I have tried running NPT calculations using the SSCHA-DFT script provided in the tutorial and also the manual submission explained here.

However in both cases, when I extracted the structure from the saved dynamical matrix, the cell parameters seemed to match the ones listed under the "New unit cell" at the end of each iteration.

From my understanding, the energies, enthalpy, and pressure in the enthalpic contribution section refer to the cell described under "[CELL] unit_cell". So I'd like to confirm if it is necessary to modify the cell parameters of the structure in the final dynamical matrix before using it for further analysis?

Thank you in advance,
Putu

[mesonepigreco]

Hi,
The NPT algorithm optimizes the simulation cell at each iteration. In the output of the final iteration, the cell provided corresponds to what the algorithm would have used in the next step. If your calculation is converged (i.e., the final stress tensor matches the target values), this updated cell should be consistent with the computed free energy. The volume associated with this new cell is, in principle, more reliable, as it benefits from one additional iteration of optimization.

If you need the cell corresponding to the exact point where the free energy was calculated, you would need to revert to the previous cell. However, if your calculation is well-converged, this change should not significantly impact your results. In fact, using the new cell is generally preferable, as it reflects a more refined estimate of the system's equilibrium state.

[PutuB]

I see, thank you very much for the explanation.