Refinement of the OPLSAA Force-Field for Liquid Alcohols

Abstract

We employ the popular all-atom optimized potential for liquid simulations, OPLSAA, force-field to model 17 different alcohols in the liquid state. Using the standard simulation protocol for few hundred nanosecond time periods, we find that 1-octanol, 1-nonanol, and 1-decanol undergo spontaneous transition to a crystalline state at temperatures which are 35–55 K higher than the experimental melting temperatures. Nevertheless, the crystal structures obtained from the simulations are very similar to those determined by X-ray powder diffraction data for several n -alcohols. Although some degree of deviations from the experimental freezing points are to be expected, for 1-nonanol and 1-decanol, the elevation of the freezing temperature warrants special attention because at room temperature, these alcohols are liquids; however, if simulated by the OPLSAA force-field, they will crystallize. This behavior is likely a consequence of exaggerated attractive interactions between the alkane chains of the alcohols. To circumvent this problem, we combined the OPLSAA model with the L-OPLS force-field. We adopted the L-OPLS parameters to model the hydrocarbon tail of the alcohols, whereas the hydroxyl head group remained as in the original OPLSAA force-field. The resulting alcohols stayed in the liquid state at temperatures above their experimental melting points, thus, resolving the enhanced freezing observed with the OPLSAA force-field. In fact, the mixed-model alcohols did not exhibit any spontaneous freezing even at temperatures much lower than the experimental values. However, a series of simulations in which these mixed-OPLSAA alcohols started from a coexistence configuration of the liquid and solid phases resulted in freezing transitions at temperatures 14–25 K lower than the experimental values, confirming the validity of the proposed model. For all of the other alcohols, the mixed model yields results very similar to the OPLSAA force-field and is in good agreement with the experimental data. Thus, for simulating alcohols in the liquid phase, the mixed-OPLSAA model is necessary for large (7 carbons and above) hydrocarbon chains.

Prediction of octanol-water partition coefficients for the SAMPL6-$$\log P$$ molecules using molecular dynamics simulations with OPLS-AA, AMBER and CHARMM force fields

“…The OPLS-AA wet octanol system underwent a phase transition to a dense solid phase during the simulation at 273 K (Figure 2a). The experimental melting temperature of pure octanol is 258.5 K and although water-octanol mixtures will have a different melting temperature, the behavior that we observed here is likely unphysical because similar behavior was recently reported and considered a consequence of exaggerated attractive interactions between the alkane chains of the alcohols in standard OPLS-AA [48]. Because we observed no phase transition in simulations at temperatures around our simulation temperature (300 K) we used the standard OPLS-AA parameters of octanol for the OPLS-AA calculations in this work.…”

Liquids relax and unify strain in graphene

“…Intra-molecular interactions within graphene flakes were described by the AIREBO potential 65 . For 1-octanol and cyclohexane two recently developed models were used that are based on the Opls forcefield [66][67][68] . These two models were specifically refined on reproducing thermodynamic properties such as the bulk density.…”

Molecular simulation study of dielectric constants of pure fluids and mixtures

“…Another likely reason for the underestimation of the dielectric constant is the neglect of polarizability, which is common to most molecular models available to date. This hypothesis is supported by the fact that several transferable force fields also underestimate the dielectric constant [21][22][23].…”

“…Núñez-Rojas et al [22] carried out a similar study on the TraPPE/UA force field and also found that the dielectric constant is typically underestimated, with an average deviation of about 37%. Zangi [23] investigated the OPLS/AA force field for alcohols, and found an average underestimation of the dielectric constant of about 33%. Including experimental data on the dielectric constant in the model parametrization has been suggested only recently [24,25].…”