docs: def finite_well_energy(a, m, U, n=1):
      '''
      a : float
          thickness of the well in units of meters
      m : float
          effective mass in the well as a fraction of the electron mass
      U : float
          the potential in eV
      n : int
          the quantum number of the desired state
  
      If U <= 0, returns 0. Otherwise, returns the confinement energy in
      units of eV.
  
      Reference:
      D.L. Aronstein and C.R. Stroud, Jr., Am. J. Phys. 68, 943 (2000).
      '''
      if U <= 0.:
          return 0
      P = prefactor * a * sqrt(m * U)
      print P, _finite_well_energy(P, n)
      return _finite_well_energy(P, n) * 7.61996e-20 / m / a ** 2.