describes the dynamics driven by the Hamiltonian, ⟨ ⟨ H ^ ^ ⟩ ⟩ n ref = Tr n { H ^ ^ ρ ρ n ref } which are Hamiltonian and Markovian in nature, while the other two terms on the right-hand side represent the non-Hamiltonian and non-Markovian dynamics that arise from the interactions between the electronic and nuclear degrees of freedom.