where E j = ⟨ ⟨ ϕ ϕ j | H | ϕ ϕ j ⟩ ⟩ is the eigen value. For the last equality, d j n = − − d n j is used. This shows that the nuclear forces acting during the hop are in the direction of the nonadiabatic coupling vector d j n . Hence d j n is a reasonable choice for the direction along which velocity should be changed.