Instantaneous normal mode analysis for intermolecular and intramolecular vibrations of water from atomic point of view

Abstract

By exploiting the instantaneous normal mode (INM) analysis for models of flexible molecules, we investigate intermolecular and intramolecular vibrations of water from the atomic point of view. With two flexible SPC/E models, our investigations include three aspects about their INM spectra, which are separated into the unstable, intermolecular, bending, and stretching bands. First, the O- and H-atom contributions in the four INM bands are calculated and their stable INM spectra are compared with the power spectra of the atomic velocity autocorrelation functions. The unstable and intermolecular bands of the flexible models are also compared with those of the SPC/E model of rigid molecules. Second, we formulate the inverse participation ratio (IPR) of the INMs, respectively, for the O- and H-atom and molecule. With the IPRs, the numbers of the three species participated in the INMs are estimated so that the localization characters of the INMs in each band are studied. Further, by the ratio of the IPR of the H atom to that of the O atom, we explore the number of involved OH bond per molecule participated in the INMs. Third, by classifying simulated molecules into subensembles according to the geometry of their local environments or their H-bond configurations, we examine the local-structure effects on the bending and stretching INM bands. All of our results are verified to be insensible to the definition of H-bond. Our conclusions about the intermolecular and intramolecular vibrations in water are given.

Local structural effects on orientational relaxation of OH-bond in liquid water over short to intermediate timescales

“…This result differs from the slope S A for the intramolecular INM bands. 67 For molecules with only accepted H-bonds (the dashed lines for n D = 0 in Fig. 12), S A was roughly 30 and 25 cm −1 for the x and y axes, respectively.…”

“…12, the average frequencies of the ten H-bond configurations are plotted against n = n D + n A for the x and y axes, with the manner of plotting being similar to that presented in Ref. 67 with n D when n was lower than four. Averaged for the linear lines fit to each data set with the same n A , the mean slope S D for the linear increase of ω μξ with n D was approximately 75 and 72 cm −1 for the x and y axes, respectively.…”

“…65, the structures of the simulated liquids were analyzed using two methods: Voronoi tessellation 79 and H-bond configuration. As in our previous studies, [65][66][67] the molecules in a configuration were classified into either four Voronoi groups (VGs) according to the asphericity of the Voronoi polyhedron associated with each molecule 80 or ten H-bond configurations. In each type of classification, a subensemble consisted of molecules belonging to a VG or with the same H-bond configuration.…”

“…[65][66][67]80 On average, VG II contained the majority of molecules, with a molecular fraction close to two thirds, and had a lifetime longer than 1 ps; the VG lifetime was estimated as the decay of the TCF of the VG index to one half. 65 VG III had a molecular fraction of approximately 30% and a lifetime near 0.24 ps.…”

“…Second, we classified water molecules into subensembles according to their local environments and the rotational stable-INM spectra of these subensembles were calculated. [65][66][67] Using the rotational stable-INM spectra of these subensembles, we could, therefore, examine the localstructure effects on the OH-bond orientational TCFs of water. The second-cumulant predictions, which were calculated using either the AVAF power spectra or the rotational stable-INM spectra, for the OH-bond orientational TCFs were highly consistent with the simulation results on short timescales.…”