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IR spectra of TIP3P water, see text. (a) Bulk harmonic HDO from classical MD at (black solid lines), 600 K (blue dashed lines), and 800 K (green dotted lines). [(b)–(e)] bulk HDO, , , and an isolated HDO molecule in the gas phase, respectively, at from classical MD (black solid lines) and CMD (red dashed lines); harmonic and anharmonic models are shown in the lower and upper subpanels, respectively.
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Probability distributions correlating the quantum expectation value (upper panel) and the centroid (lower panel) of the OH bond length with the protonic radius of gyration for one harmonic molecule with at 100 K in the gas phase. The probabilities grow from yellow to red to black and the horizontal dashed lines mark the corresponding equilibrium bond length . Insets illustrate very schematically in 2D the qualitative behavior of a covalent OH bond in at high (left) and low (right) temperatures, see text.
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Centroid molecular dynamics (CMD) is a popular method to extract approximate quantum dynamics from path integral simulations. Very recently we have shown that CMD gas phase infrared spectra exhibit significant artificial redshifts of stretching peaks, due to the so-called “curvature problem” imprinted by the effective centroid potential. Here we provide evidence that for condensed phases, and in particular for liquid water, CMD produces pronounced artificial redshifts for high-frequency vibrations such as the OH stretching band. This peculiar behavior intrinsic to the CMD method explains part of the unexpectedly large quantum redshifts of the stretching band of liquid water compared to classical frequencies, which is improved after applying a simple and rough “harmonic curvature correction.”
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