SEARCH Articles Figures Tables Approximations order Boltzmann equation first-order approximation Crude Born-Oppenheimer approximation first-order derivatives Determination of Spectra from Viscoelastic Functions Using First-Order Approximations First-Order Regular Approximation First-Order Regular Approximation method First-order absorption models approximation with First-order approximation differential equations First-order approximation, relaxation First-order approximation, relaxation spectra First-order polarization propagator approximation Matrix first-order approximation Pseudo-first order approximation Steady states first-order approximation