On the use of the finite-element method with Lagrange multipliers in scattering theory

It is shown that the use of the finite-element method with Lagrange multipliers in scattering theory is equivalent to the Kohn variational principle. For simple test problems, the convergence of the reactance matrix is found to agree with the predicted behaviour, that is, the error in K ij is proportional to the square of the error of the Lagrange multiplier.