for that region simple. In the MT spheres this setup is also simple and computationally inexpensive for the kinetic energy and the spherically averaged potential, e.g., in the muffin-tin approximation. The simplicity hereby stems from the connection of the radial functions to the spherical Hamiltonian in the spheres H ^ ^ sphr α α , i.e., H ^ ^ sphr α α | u l , α α ( r α α , E l , α α ) ⟩ = E l , α α | u l , α α ( r α α , E l , α α ) ⟩ and H ^ ^ sphr α α | u ˙ ˙ l , α α ( r α α , E l , α α ) ⟩ = E l , α α | u ˙ ˙ l , α α ( r α α , E l , α α ) ⟩ + | u l , α α ( r α α , E l , α α ) ⟩ . In comparison to the MT approximation, for the full-potential description (FLAPW) contributions from the non-spherical part of the potential are added to the Hamiltonian matrix in the MT spheres and in the IR contributions related to deviations from the constant potential.