The helium atom consists of two electrons with mass m and electric charge − e, around an essentially fixed nucleus of mass M ≫ m and charge +2 e. The Hamiltonian for it, neglecting the fine structure, is: H = − − ℏ ℏ 2 2 m ( ∇ ∇ 1 2 + ∇ ∇ 2 2 ) − − e 2 4 π π ε ε 0 ( 2 r 1 + 2 r 2 − − 1 | r 1 − − r 2 | ) where ħ is the reduced Planck constant, ε 0 is the vacuum permittivity, r i (for i = 1, 2) is the distance of the i -th electron from the nucleus, and | r 1 − r 2 | is the distance between the two electrons.