Here, P 1,2 i denotes the populations of the molecules on the unselected branch. According to the notation introduced in the Hamiltonian Ĥspath , for instance, P 1 i = ρ11(t) and P 2 i = ρ33 (t) when the intended branch is b, while P 1 i = ρ22 (t) and P 2 i = ρ44 (t) apply when branch a is the target.