Extended overlap distribution method for computation of difference spectroscopy

Difference spectroscopy between two similar related spectra is routinely applied to highlight features associated with the difference, although computation of such a small difference spectrum by subtracting two spectra is quite challenging due to its huge sampling cost. To overcome this challenge, we have developed a series of new theories that enabled direct calculation of minute difference spectra under background-free conditions, which achieved remarkable improvement of computational efficiency. This work reports our further advances in the theory of difference spectroscopy with the aid of the overlap distribution method of free energy calculations. The present work resolved the main remaining problem of computation in the cases involving different excluded volume regions and thus greatly expanded the applicability of the theoretical analysis to a wide range of systems. We applied the revised theory to the hydrophobic hydration processes and explored spectral features of various association states in the infrared spectrum of water. The present theory paves the way for practical computational analysis of a variety of difference spectroscopic techniques.