An Extended Mixed Quantum/Classical Approach for Quantitative Calculation of Complex Refractive Index

The mixed quantum/classical approach of Skinner and co-workers has been widely used to calculate the line shapes of the infrared spectra of water (H₂O), but less attention has been paid to the use of this approach in quantitatively calculating spectral intensity, thereby limiting direct comparisons of calculated and experimental spectra. Here, we extend this theoretical framework to facilitate direct computation of the full complex refractive index of water, replacing the normalized ordinate used in previous studies. Our results for the OH stretching region of H₂O capture both the shapes and intensities of the experimental spectra. They reveal that inclusion of the local field effect is crucial to the accurate reproduction of spectral intensity. This extended approach enables new areas of analysis of the bulk, thin-film, and cluster spectra of water.