Higher-order response theory in optimal stochastic thermodynamics

Linear response theory has found many applications in statistical physics. One of these is to compute minimal-work protocols that drive nonequilibrium systems between different thermodynamic states, which are useful for designing engineered nanoscale systems and understanding biomolecular machines. We compare and explore the relationships between linear-response-based approximations used to study optimal protocols in different driving regimes by showing that they arise as controlled truncations of a general causal response (Volterra) expansion. We then construct higher-order response terms and discuss the drawbacks and utility of their inclusion. We illustrate our results for an overdamped particle in a harmonic trap, ultimately showing that the inclusion of higher-order response in calculating optimal protocols provides marginal improvement in effectiveness despite incurring a significant computational expense, while introducing the possibility of predicting arbitrarily low and unphysical negative excess work.