In many systems, one can consider a perturbing field h that couples linearly to the order parameter. For example, in the case of a classical dipole moment μ μ , the energy of the dipole-field system is − − μ μ B . In the general case, one can assume an energy shift of − − η η h due to the coupling of the order parameter to the applied field h , and the Landau free energy will change as a result: