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 Dear Andy, When you have the potential, the electric field can be obtained as a post-treatment since it is (minus) the gradient of the electric potential. You can simply interpolate it with ```E = gf_model_get(md, 'interpolation', "-Grad_V', mfi); ``` once you solved your problem (mfi is the fem on which the interpolation is performed). You have to be aware that the gradient of the potential being discontinuous across the element edges for Lagrange elements, an average is taken when interpolating on a continuous Lagrange fem (you can interpolate on a discontinuous one to have the true gradient).   Yves. Le 09/06/2017 à 04:55, Yu (Andy) Huang a écrit : Dear getFEM users, I'm entirely new to getFEM, and I'm trying to simulate the electric field distribution in the human brain when direct electric current is applied on the scalp surface. I know it's just a Laplacian equation of the electric potential, and I managed to simulate the voltage distribution on a toy (a cube).  Now my question is: how do I simulate the electric field? should I add another variable of electric field? or can I just get the field from the voltage solution? I tried both but without any luck. I added the electric field as a new variable but did not figure out how to properly add boundary condition using gf_model_set(). If calculating field from voltage, I didn't find out which function to use to establish a relation between the field variable and voltage variable. Any suggestion is appreciated! The examples in the documentation are generally mechanical problems, and there are very limited online resources, so I really get stuck here. Thanks a lot! -- Yu (Andy) Huang, Ph.D. Postdoc fellow at Dept. of Biomedical Engineering, City College of New York Center for Discovery and Innovation, Rm. 3.320, ```-- Yves Renard (address@hidden) tel : (33) 04.72.43.87.08 Pole de Mathematiques, INSA-Lyon fax : (33) 04.72.43.85.29 20, rue Albert Einstein 69621 Villeurbanne Cedex, FRANCE http://math.univ-lyon1.fr/~renard --------- ```