Here, ℏ ℏ is the reduced Planck constant, ∇ ∇ is the nabla operator, μ μ is the reduced mass, − − | e | ( + | e | ) is the elementary charge related to an electron (hole), ε ε r is the relative permittivity, and ε ε 0 is the vacuum permittivity. The solutions of the hydrogen atom are described by eigenfunction ϕ ϕ λ λ ( r ) and eigenenergy E λ λ where λ λ is a quantum number labeling the different states.