Removing the barrier to the calculation of activation energies

Approaches for directly calculating the activation energy for a chemical reaction from a simulation at a single temperature are explored with applications to both classical and quantum systems. The activation energy is obtained from a time correlation function that can be evaluated from the same molecular dynamics trajectories or quantum dynamics used to evaluate the rate constant itself and thus requires essentially no extra computational work

Removing the barrier to the calculation of activation energies: Diffusion coefficients and reorientation times in liquid water

See also: The Journal of Chemical Physics 145 (13), 134107 (2016). The following article appeared in Piskulich, Z. A., Mesele, O. O., & Thompson, W. H. (2017). Removing the barrier to the calculation of activation energies: Diffusion coefficients and reorientation times in liquid water. The Journal of Chemical Physics, 147(13), 134103. and may be found at https://aip.scitation.org/doi/10.1063/1.4997723.General approaches for directly calculating the temperature dependence of dynamical quantities from simulations at a single temperature are presented. The method is demonstrated for self-diffusion and OH reorientation in liquid water. For quantities which possess an activation energy, e.g., the diffusion coefficient and the reorientation time, the results from the direct calculation are in excellent agreement with those obtained from an Arrhenius plot. However, additional information is obtained, including the decomposition of the contributions to the activation energy. These results are discussed along with prospects for additional applications of the direct approach

A “Universal” Spectroscopic Map for the OH Stretching Mode in Alcohols

Empirical maps are presented for the OH stretching vibrations in neat alcohols in which the relevant spectroscopic quantities are expressed in terms of the electric field exerted on the hydrogen atom by the surrounding liquid. It is found, by examination of the four lowest linear alcohols, methanol, ethanol, <i>n</i>-propanol, and <i>n</i>-butanol, that a single map can be used for alcohols with different alkyl groups. This “universal” map is in very good agreement with maps optimized for the individual alcohols but differs from those previously developed for water. This suggests that one map can be used for all alcohols, perhaps even those not examined in the present study. The universal map gives IR lineshapes in good agreement with measured spectra for isotopically dilute methanol and ethanol, while the two-dimensional IR photon echo spectra give results that differ from experiments. The role of non-Condon effects, reorientation dynamics, hydrogen bonding, and spectral diffusion is discussed