Improved Theory of Difference Vibrational Spectroscopy and Application to Water

Measuring difference spectrum of two closely related systems is a versatile experimental method to highlight the difference in the two systems. However, the computation of a minuscule difference between two spectra by molecular dynamics (MD) simulation is far more challenging than that of each spectrum in terms of statistical convergence. Therefore, we have proposed a theory of difference spectra, which accelerates the calculation of difference spectra by several orders of magnitude [ Sakaguchi, S. et al. J. Chem. Phys. 2014 , 140 , 144109 ]. The present paper reports our subsequent advances in the computational method to greatly improve its accuracy, numerical stability, and applicability. The present method of computation based on the nonequilibrium MD simulation allows for general molecular models including polarizable ones without sacrificing its computational efficiency. The improved method was applied to polarizable liquid water and yielded difference spectra of infrared, Raman, and vibrational sum frequency generation spectroscopies. The present method enables us to analyze various difference spectra of large molecular systems using MD simulation.