Simulations of the OKE Response in Simple Liquids Using a Polarizable and a Nonpolarizable Force Field

Abstract

Recently polarizable force fields are becoming increasingly popular for molecular dynamics simulations. As the signal obtained in the optical Kerr effect (OKE) experiment is due to the polarizability dynamics of the investigated system, a study is conducted in order to compare the experimental results with those obtained with the polarizable AMOEBA force field. The comparison is made in the frequency domain; however, time domain data are also included. The selected molecular systems are the isotropic carbon tetrachloride molecule, the anisotropic chloroform, carbon disulfide and acetone molecules, and the hydrogen-bonded water and methanol molecules. Different dipole-induced-dipole (DID) method variants are used for calculation of the OKE response, showing the importance of use of the all-atom approach with preoptimized atomic polarizabilities. In order to obtain a good intermolecular to intramolecular components amplitude ratio, the isotropic polarizability in the Thole correction needs to be updated between iterations. The convergence of the spectra calculated with different DID variants is also considered, and the approach that appears to be the best gives a very good approximation after three iterations. The comparison of the experimental and simulated spectra shows a rather good agreement for the non-hydrogen-bonded molecules, although the contribution of the reorientation of anisotropic molecules is overestimated. In the case of the hydrogen-bonded molecules, the theoretical spectra are far from the experimental ones. The highly overestimated librational bands indicate excessive polarizability anisotropy introduced by the potential model. Finally, in order to verify the significance of different components of the AMOEBA model, it is gradually simplified and compared with a simple reference potential model. Removal of polarizability shows a tremendous change in the case of hydrogen-bonded liquids, whereas for the other molecules it is of minor importance. The non-hydrogen-bonded liquids are, however, more sensitive to the presence of atomic multipoles in the model.

Charge Gradients around Dendritic Voids Cause Nanoscale Inhomogeneities in Liquid Water

“…Molecular dynamics simulation studies by others, however, severely contest these interpretations and insist that water is a homogeneous liquid. ,,, To understand the structure of water on the molecular and nanoscale level, there is a need for alternative experimental techniques that can probe the water structure over nanometric length scales and, correspondingly, on time scales comparable to the femtosecond restructuring time of the hydrogen bond network of water. However, although there are many spectroscopic techniques to probe the water structure, − nearly all methods are sensitive to the hydration shell (subnanometric length scales involving at most a few layers of water) and spatiotemporal structural averaging, severely limiting our understanding of the nanometric length and femtosecond time scales on which water structures and transforms …”

Protein–Ligand Binding Molecular Details Revealed by Terahertz Optical Kerr Spectroscopy: A Simulation Study

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Anisotropic and Finite Effects on Intermolecular Vibration and Relaxation Dynamics: Low-Frequency Raman Spectroscopy of Water Film and Droplet on Graphene by Molecular Dynamics Simulations

“…The former α i M is represented by the gas phase molecular polarizability, and the latter α i I based on the dipole–dipole coupling interactions between molecules is obtained by solving a set of self-consistent equations In eq , μ i and β i M are dipole moment and molecular hyperpolarizability of molecule i , respectively, and T ij is the dipole-interaction tensor between molecule i and j , with r ij the vector between the center of mass of molecules i and j separated at a distance r ij and I the 3 × 3 unit tensor. The second term in the RHS of eq , which is called the extended dipole-induced-dipole mechanism, describes the enhancement of the molecular polarizability due to the dipole field ∑ j (≠ i ) T ij μ j on molecule i . − The dipole μ i enhanced due to the dipole-induced-dipole mechanism were determined by solving the following equations Thus, the total molecular and induced polarizabilities are expressed by the corresponding polarizabilities of the molecules as follows Note that we take into account up to the first-order term with respect to the electric dipole field in the induced dipole moment (eq ) and polarizability (eq ). These eqs (eqs and ) were solved iteratively by a simple successive approximation method at each configuration stored every 10 fs.…”