The problems about the structures in poscar when appling an external electric field.

[xinhuanwen]

Hello, everyone
I have encountered some problems puzzing me all the time when appling an external electric field. I want to repeat the outcome in Nature 444, 347 (2006). (Here, http://www.nature.com/nature/journal/v4 ... 05180.html) It used the SIESTA code.
I adopted the same cell and width for a zigzag graphene ribbon as the letter in Nature. Under an external electric field of 0.1 eV/A perpendicular to the zigzag edge in the ribbon plane, when the atoms composing the graphene ribbon was at the middle of the cell(especially the direction of the electric field), the scf was not converged(If the tag of DIPOL was set as 0.5 0.5 0.5 in INCAR, calculation gets converged). If the zigzag graphene nanoribbon is not at the middle of the cell in the direction of the electric field, the scf is converged, but it can not give the half-metal result. The calculations above are all spin-polarized.
I want to know the relationship among these inputs: structures in POSCAR, the tag of DIPOL, and the electric field.

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