An error "Singular matrix" occurs.
The system matrix (Jacobian matrix or stiffness matrix) is singular, so the solver cannot invert it. Usually this means that the system is underdetermined. Check that all equations are fully specified and that the boundary conditions are appropriate. For instance, in a stationary model you usually need to have a Dirichlet condition on some boundary (that is, specifying the value of the solution on the boundary).
A singular matrix could also occur if mesh elements are of too low quality. If the minimum element quality is less than 0.005 you might be in trouble (to check the mesh quality, plot the built-in variable
qual). Another reason for this error message is that you have different element orders for two variables that are coupled by, for example, a weak constraint. Use the same element order for all variables that are coupled.
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