[Q-e-developers] About differiential calculation

[Jiqiang Li]

Dear developers,
        I want to ask a trivial question about total local potential Vtot (V_Hartree + Vxc + V_ion), i.e., the variable vrs in electron.f90 (correct me if I am wrong). When we move a specific atom along a specific direction, does the derivative quantity dVtot/dR remain unchanged ?
       I try 0.01~0.1 Angstrom as the amplitude of the displacements, and find that dVtot/dR is not independent of dR. does the differece arise from the numerical error or theoretically should dVtot/dR depend on dR? If the former is right, then how long is the optimal value of dR for this calculation generally?

Best regards!

Jiqiang Li
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