Calculation of momentum matrix elements using the Green's-function method

A method for calculating the interband momentum matrix elements in the Green's-function method has been obtained, which is applicable to the $l$- and $E$-dependent potentials. This method is convenient in that it requires only the knowledge of wave functions inside the muffin-tin spheres. Numerical results calculated from this method are used to study the dependence of the matrix elements on the number of angular momentum components included. It is found that $l>\mathrm{}2\mathrm{}$ components are important for the matrix elements.