After writing the wave function as a product of functions: ψ ψ ( r , θ θ , ϕ ϕ ) = R n l ( r ) Y ℓ ℓ m ( θ θ , ϕ ϕ ) (in spherical coordinates), where Y ℓ ℓ m are spherical harmonics, we arrive at the following Schrödinger equation: − − ℏ ℏ 2 2 μ μ [ 1 r 2 ∂ ∂ ∂ ∂ r ( r 2 ∂ ∂ R ( r ) ∂ ∂ r ) − − l ( l + 1 ) R ( r ) r 2 ] + V ( r ) R ( r ) = E R ( r ) , where μ μ is, approximately, the mass of the electron (more accurately, it is the reduced mass of the system consisting of the electron and the nucleus), and ℏ ℏ is the reduced Planck constant.