[QE-users] relaxation of a charged system

[Hanghui Chen]

Dear QE developers,
     I have been using Quantum Espresso for more than a decade and recently
a paper (PRB 91, 024107 (2015)) caught my attention, which shows that the
total energy, pressure and cell relaxation are ill-defined in charged
systems (even if a compensating charge background is present). The paper
shows that the total energy and pressure of a charged system depend on the
average electrostatic potential convention, and thus is code-dependent (QE,
ABINIT and VASP use different conventions).
    I think the paper is theoretically correct. However in practice, we
have recently tested doped ferroelectric materials using both QE and VASP.
In this charged system, we find both QE and VASP yield qualitatively
consistent structural properties.
    Since relaxation of a charged system is very common in literature using
QE or VASP (e.g. PRL 109, 247601 (2012) and many others), I am wondering
whether the relaxed structure of a charged system from QE is reliable.
According to PRB 91, 024107, it is not legitimate to do cell relaxation of
a charged system. However both QE and VASP do allow to calculate total
energy and perform cell relaxation in a charged system. And from
literature, it seems that the relaxed structure of a charge system from QE
or VASP calculations is "not too wrong".
    So what is your insight on this issue? Shall we prohibit ourselves from
doing cell relaxation of a charged system in QE, or can we still use the
standard procedure but with the caution that the optimized structure of a
charged system from QE might not be accurate (due to the QE convention of
average electrostatic potential)?
     Thank you very much.

Dr. Hanghui Chen
Department of Physics
NYU Shanghai and New York University
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