Book excerpt: The pure rotational and vibrational-rotational absorption bands for a diatomic are calculated directly from classical molecular dynamics, classical linear response theory and classical statistical mechanical ensemble averaging with the use of simple quantum corrections. The experimental spectral band intensities and contours are well reproduced for CO from dilute gas phase through solution in compressed Ar to solution in liquid Ar by these 'Newtonian' classical spectral calculations. The typical evolution seen in vibrational spectra from multiple-peaked gas phase bands to single-peaked solution bands is observed. The 'Newtonian' gas phase calculations also match quantum and correspondence principle classical spectral calculations. This molecular dynamic approach may be applied to compute the spectra of complex molecules or liquids for which a normal mode analysis may be impractical, and may also be extended to nonequilibrium systems, for example to compute transient vibrational spectra during chemical reactions. (Author).