which is purely diagonal due to the diagonal Ĥsb in the diabatic basis (Eq. 22-Eq. 24). For the trajectory surface hopping approach, the nuclear force is F = -∇ R ⟨J(R)|( Ĥsb + Ĥb )|J(R)⟩, where |J(R)⟩ is an active adiabatic surface (an eigenstate of ĤQ ) that is determined stochastically through a given algorithm (such as the most widely used fewest switches algorithm ).