import fipy as fp

nx = 20
ny = nx
dx = 1
dy = dx
mesh = fp.Grid2D(dx=dx, dy=dy, nx=nx, ny=ny)

phi = fp.CellVariable(mesh=mesh, value=0.)
eq = fp.TransientTerm() == fp.DiffusionTerm(coeff=1)

eq.solve(var=phi, dt=1.)

raises

• RuntimeError: Factor is exactly singular
• StagnatedSolverWarning: The solver stagnated. Iterations: 2. Relative error: nan
• RuntimeWarning: invalid value encountered in true_divide

depending on solver.

dx = 1. fixes this.