Staff editor
i2=−1i^2=-1i2=−1. It makes the generator of time evolution unitary rather than dissipative.
Turns the Hamiltonian into a phase rotation of Ψ\PsiΨ. Without iii, probability would not be conserved.
The factor iii is why stationary states pick up a phase e−iEt/ℏe^{-iEt/\hbar}e−iEt/ℏ instead of growing or decaying.