protected symbols used internally by discretizer
Kind of important question: shall we forbid user from using some symbols we used internally: like Psi
for wave function or ax, ay, az = sympy.symbols('a_x a_y a_z')
that we are using for doing derivations.
However a
itself stays for lattice constant I wouldn't forbid providing hamiltonian with it, for example
H = sympy.Matrix([[kx*A*kx, sympy.sin(x/a)*kx], [kx*sympy.sin(x/a), C]])
looks like perfectly fine input for me.