Force Fields with Fixed Bond Lengths and with Flexible Bond Lengths: Comparing Static and Dynamic Fluid Properties

Abstract

This study investigates the equivalence or differences between classical force fields with rigid bond lengths and the same models but with (harmonic) bond length potentials. For ethane, propane, and dimethyl ether described with the Transferable Anisotropic Mie potential, we vary the force constant of the harmonic bond length potentials and analyze static and dynamic physical properties, namely pressure, viscosity, self-diffusion, and thermal conductivity of homogeneous phases. We find a range of values for the force constant of the bond length potentials (expressed in terms of the period-length of bond-oscillations) where force fields with harmonic bond lengths give equivalent results as the model with rigid bond lengths for static properties, for viscosity, and for self-diffusion coefficients. The thermal conductivity of the force field with rigid bond lengths has an offset compared to the harmonic bond length models, which can be approximated through an analytic correction term. After adding the correction term, results of the rigid model and the flexible models are in rather close agreement. Our study varies time-steps for solving the equations of motion and investigates whether the rRESPA integrator with a small time step associated to the (rather high frequency) bond length potentials has advantages compared to a simple velocity Verlet integrator. Furthermore, this work proposes a fast and memory-efficient prescription to calculate autocorrelation functions for the calculation of Green–Kubo integrals. We then estimate average values and meaningful error bars for dynamic physical properties based on the time-decomposition approach Zhang, Y.; Otani, A.; Maginn, E.J.: Reliable Viscosity Calculation from Equilibrium Molecular Dynamics Simulations: A Time Decomposition Method, J. Chem. Theory Comput. 2015, 11, 3537–3546.