Perturbative valence charge density of trigonal Se and Te

The electron valence charge density in Se and Te is calculated using perturbation theory to second order in an average form factor which accounts for the lion-locality of the pseudo-potential. The convergence of the perturbative expansion is tested and discussed, and a comparison is made with previous calculations derived from band structures. The importance of the second order term is related to the covalence of the chemical bond, lattice dynamics, and bonding charge models.