Hi all.<div><br></div><div>I have already asked my question at&nbsp;<a href="http://math.stackexchange.com/questions/422928/modeling-gravity-field-with-finite-elements">http://math.stackexchange.com/questions/422928/modeling-gravity-field-with-finite-elements</a>&nbsp;and&nbsp;<a href="http://stackoverflow.com/questions/17160835/modeling-gravity-field-with-finite-elements">http://stackoverflow.com/questions/17160835/modeling-gravity-field-with-finite-elements</a>&nbsp;but seems there are no people there, who knows sfepy and finite elements, in particular.</div><div><br></div><div>I want to model the gravity field on 3D rectangular area. It can be described by the equation: div(G) = rho. Here G is vector unknown function, rho is scalar parameter, which is constant&nbsp;at the selected point and fully determined by material.</div><div><br></div><div>The weak form is: int( div(G) * g ) = int( rho * g ). Here g is vector test function.</div><div>Using sfepy syntax: {'Gravity' : """dw_div_grad.1.Omega( g, G ) = dw_volume_lvf.1.Omega( m.rho, g )"""}</div><div><br></div><div>I used gmsh to build .mesh file. My .geo, .mesh and .py files attached to this message.</div><div><br></div><div>When I try to run sfepy, it says that matrix is singular, so the&nbsp;solving process does not converge.</div><div><br></div><div>What I have to do to correct the model?<br></div><div><br></div><div>Thanks in advance.</div><div><br></div>