Nuclear quantum effects on the vibrational dynamics of liquid water

Abstract

Based on quantum-mechanical path-integral molecular dynamics simulations, the impact of nuclear quantum effects on the vibrational and hydrogen bond dynamics in liquid water is investigated. The instantaneous fluctuations in the frequencies of the O-H stretch modes are calculated using the wavelet method of time-series analysis, while the time scales of the vibrational spectral diffusion are determined from frequency-time correlation functions, joint probability distributions, and the slope of three-pulse photon echo. We find that the inclusion of nuclear quantum effects leads not only to a redshift of the vibrational frequency distribution by around 130 cm but also to an acceleration of the vibrational dynamics by as much as 30%. In addition, quantum fluctuations also entail a significantly faster decay of correlation in the initial diffusive regime, which is in agreement with recent vibrational echo experiments.

Impact of intermolecular vibrational coupling effects on the sum-frequency generation spectra of the water/air interface

“…The impact of NQE can be clearly seen in Figure 6, showing that the inclusion of NQE leads to a blue-shift of ∼ 50 cm −1 with respect to experimental SFG measurements, whereas neglecting NQE entails a red-shift of ∼ 150 cm −1 . This is to say that NQE makes up for a blue-shift of as much as ∼ 200 cm −1 that is a well known phenomenon and have already been reported by previous studies [30,[56][57][58]. Yet, neglecting NQE, non-Condon or many-body effects are necessary to obtain good agreement with experimental SFG measurements in terms of the intensities within the H-bonded O-H peak region [11,30].…”

“…To explicitly consider nuclear quantum effects (NQE) in a computational efficient way, the ring polymer contraction scheme was used with a cutoff value of σ = 5 Å to reduce the electrostatic potential energy and force evaluations to a single Ewald sum, in order to speed up the calculations [52]. This is to say that 32 ring-polymer beads were employed to converge all relevant properties [47,52,[56][57][58][59][60], while the computationally expensive part of the electrostatic interactions were contracted to the centroid only. In all simulations, using a discretised integration timestep of 0.1 fs, the system was first equilibrated 10 ps in the canonical ensemble, before microcanonical ensemble averages were computed over the following 8 ps, resulting in a total statistics of 2 ns to compute the present SFG spectra.…”

Time-dependent vibrational sum-frequency generation spectroscopy of the air-water interface

“…We note that this scheme requires a priori knowledge of the vibrational frequency of the OH mode. The time-dependent fluctuations in the vibrational frequencies of the OH modes are generally determined using empirical maps 29,30,43,44 , or a short-time wavelet transform [54][55][56] . In the present work, we have employed an empirical linear relationship between the hydrogen bond strength and its vibrational frequency, as reported in an earlier work by some of us 57 , which reads as…”

“…However, we note that in our force-field-based simulation study, we have not included the nuclear quantum effects (NQE). The inclusion of NQE not only leads to an overall red-shift of $100 cm À1 within the vibrational frequency distribution, but also an excellent recreation of the nonlinear vibrational spectrum 51,56 . In Fig.…”

Tumbling with a limp: local asymmetry in water's hydrogen bond network and its consequences

“…Equilibrium distributions of OH stretch frequency versus HB strength, as quantified by the ∆E CT terms (left) and the associated chargetransfer terms (∆Q CT in units of elementary charge, right). The frequencies of the OH modes were calculated using a wavelet-based time-series analysis [40][41][42]. Reproduced from Ref.…”