get a better example model than the XXZ Hamiltonian (which is used for many tests, where a RBasis is needed); also the fact that the XXZ Hamiltonian has discrete magnetization plateaus and is very sensitive to the degenerate snapshot at $(\Delta, h/J) = (-1, 0)$ makes this model problematic at larger $L$
write tests for untested code parts
design an artificial parametrized eigenvalue problem, where we know the exact solution (non related to quantum spin systems)
Possible improvements:
RBasis
is needed); also the fact that the XXZ Hamiltonian has discrete magnetization plateaus and is very sensitive to the degenerate snapshot at $(\Delta, h/J) = (-1, 0)$ makes this model problematic at larger $L$