I am using an FEM code written in Fortran which I did not design. For a particular problem, the program complains that the determinant of the Jacobian matrix is inferior to zero.
I vaguely understand that the Jacobian matrix is related to the shape functions and that it must be inverted to map shape function coordinates to global coordinates. To invert the matrix it must compute the determinant. Therefore I suspect this is a mesh problem.
Could someone provide added insight into this? What conditions in an FE problem would typically lead to a Jacobian determinant that is less than zero? Why is this a problem? How can it be avoided?