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I am investigating the potential energy map of a periodic array of water molecules as a function of rotation. As part of that, I am attempting to determine the partial charges (to determine the dipole) using DDEC6 via Chargemol as a post-processing step (i.e. I already have converged WAVECAR files).

In order to implement this method, I need to include the core electrons and produce the relevant AECCAR* files by running VASP with the tags "NSW = 0; LAECHG=.TRUE." in my INCAR file according to numerous tutorials/discussions. In doing so, an energy relaxation is still performed, and I am getting different energy values (as show in the attached figure)

In the figure, the red line represents single-point energy calculations for different rotations. (Here, 1 rotation = 0.1 rad.) The blue line represents the single-point calculations when running VASP again with only the above mentioned INCAR tags as a post-processing step. Aside from their absolute value difference, qualitatively the potential maps differ as well.

Is this suppose to happen? Should I get such drastic energy differences when including core elections? On the one hand, I can see how explicitly including the core elections in a wavefunction (i.e. WAVECAR file) that previously omitted them would require some relaxation, but then this also suggests that the use of pseudopotentials is not justified (an obvious contradiction).

Potential energy map of periodic array of water molecules as function of rotation with (red) and without (blue) explicitly treating the core electrons.

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Though i never used Chargemol but you query is related to VASP run, i think, in my experience, if you do convergence ISIF=3, before doing NSW = 0; LAECHG=.TRUE., then these energies will change. Is this the situation in your case?

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