A quantum chemical method for calculating vibrational line shifts in diatomic fluids

We introduce a simple procedure for generating spectral line shifts in diatomic fluids as a function of pressure. From O(100) configurations (of 100 N2 molecules) sampled in an isothermal–isobaric ensemble, forces computed with density functional theory are used to generate force–displacement correlations at a series of fluid densities. The curves are fitted with second-degree polynomials, and the resulting coefficients are related to fundamental frequencies and anharmonicities through a truncated expansion of the Morse potential. Comparison to coherent anti-Stokes Raman data reveals satisfactory agreement for line shifts as a function of pressure.