Hi Galvin, Examining the energies and stresses you obtain from your calculations by the eye is not a very accurate way to obtain the lattice parameter that minimizes the energy and corresponds to a zero pressure. More accurate is to fit your results to an equation of state such as Birch-Murnaghan or Vinet. The reason is that most likely the optimal lattice parameter lies in between the values you obtained from your scf results. As a bonus you get also other useful parameters form the fit like the bulk modulus. However, make sure that all the data points you use in the fit correspond to hydrostatic stress by examining the stress tensor printed in output file. This is because all these equation of states assume hydrostatic stress in their derivation.
Mostafa MIT
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