A tighter upper bound can be found by using a better trial wavefunction with 'tunable' parameters. Each electron can be thought to see the nuclear charge partially "shielded" by the other electron, so we can use a trial wavefunction equal with an "effective" nuclear charge Z < 2 : The expectation value of H in this state is: ⟨ H ⟩ = [ − − 2 Z 2 + 27 4 Z ] E 1