An electron in an atom or ion, placed in vacuum, is known to interact with the nucleus via the Coulomb potential. When the atom is placed in an external environment, this potential changes and as a result the energy levels and other properties of the atom change considerably. This change in the pure Coulomb potential depends on the nature of interaction of the atom with the external environment and can be approximated by model potential. As a notable example, the potential of a test charge in a weakly coupled plasma can be modelled by the Debye–H¨uckel model or screened Coulomb potential