Bottom-up approach to represent dynamic properties in coarse-grained molecular simulations.

Several molecular coarse-graining methods have been proposed in recent years to derive chemical- and state-point transferable force fields. While these force fields describe structural and thermodynamic properties in good agreement with fine-grained models and experiments, dynamic properties are usually overestimated. Herein, we examine if the long-time dynamic properties of molecular coarse-grained (CG) systems can be correctly represented by employing a dissipative particle dynamics (DPD) thermostat, which is "bottom-up informed" by means of a variant of the Markovian Mori-Zwanzig (MZ) DPD coarse-graining method. We report single-site and multiple-site CG models for a monomer, dimer, and 24mer based on 2,2-dimethyl propane as a chemical repeat unit and report data obtained from MZ-DPD simulations of liquids, polymer solutions, and polymer melts. We find that despite incomplete time scale separation of the molecular CG model, MZ-DPD achieves quantitative accuracy in predicting diffusive dynamics in single-component liquids and polymer solutions (24mers in a dimer solvent). We also find that MZ-DPD simulations of molecular penetrant diffusion in polymer networks do not reach quantitative agreement with the fine-grained model. Modeling diffusion governed by the activated barrier crossing of small molecular penetrants in these dense systems requires an accurate description of energy barriers, presumably combined with the treatment of memory effects. The use of a MZ-DPD thermostat extends the scope and applicability of molecular CG models for multicomponent systems where a correct description of the relative diffusion rates of the different components is important.

Solid-liquid work of adhesion of coarse-grained models of n-hexane on graphene layers derived from the conditional reversible work method.

We address the question of how reducing the number of degrees of freedom modifies the interfacial thermodynamic properties of heterogeneous solid-liquid systems. We consider the example of n-hexane interacting with multi-layer graphene which we model both with fully atomistic and coarse-grained (CG) models. The CG models are obtained by means of the conditional reversible work (CRW) method. The interfacial thermodynamics of these models is characterized by the solid-liquid work of adhesion WSL calculated by means of the dry-surface methodology through molecular dynamics simulations. We find that the CRW potentials lead to values of WSL that are larger than the atomistic ones. Clear understanding of the relationship between the structure of n-hexane in the vicinity of the surface and WSL is elucidated through a detailed study of the energy and entropy components of WSL. We highlight the crucial role played by the solid-liquid energy fluctuations. Our approach suggests that CG potentials should be designed in such a way that they preserve the range of solid-liquid interaction energies, but also their fluctuations in order to preserve the reference atomistic value of WSL. Our study thus opens perspectives into deriving CG interaction potentials that preserve the thermodynamics of solid-liquid contacts and will find application in studies that intend to address materials driven by interfaces.

Bottom-up derivation of conservative and dissipative interactions for coarse-grained molecular liquids with the conditional reversible work method.

Molecular simulations of soft matter systems have been performed in recent years using a variety of systematically coarse-grained models. With these models, structural or thermodynamic properties can be quite accurately represented while the prediction of dynamic properties remains difficult, especially for multi-component systems. In this work, we use constraint molecular dynamics simulations for calculating dissipative pair forces which are used together with conditional reversible work (CRW) conservative forces in dissipative particle dynamics (DPD) simulations. The combined CRW-DPD approach aims to extend the representability of CRW models to dynamic properties and uses a bottom-up approach. Dissipative pair forces are derived from fluctuations of the direct atomistic forces between mapped groups. The conservative CRW potential is obtained from a similar series of constraint dynamics simulations and represents the reversible work performed to couple the direct atomistic interactions between the mapped atom groups. Neopentane, tetrachloromethane, cyclohexane, and n-hexane have been considered as model systems. These molecular liquids are simulated with atomistic molecular dynamics, coarse-grained molecular dynamics, and DPD. We find that the CRW-DPD models reproduce the liquid structure and diffusive dynamics of the liquid systems in reasonable agreement with the atomistic models when using single-site mapping schemes with beads containing five or six heavy atoms. For a two-site representation of n-hexane (3 carbons per bead), time scale separation can no longer be assumed and the DPD approach consequently fails to reproduce the atomistic dynamics.

Thermodynamic transferability of coarse-grained potentials for polymer-additive systems.

In this work we study the transferability of systematically coarse-grained (CG) potentials for polymer-additive systems. The CG nonbonded potentials between the polymer (atactic polystyrene) and three different additives (ethylbenzene, methane and neopentane) are derived using the Conditional Reversible Work (CRW) method, recently proposed by us [Brini et al., Phys. Chem. Chem. Phys., 2011, 13, 10468-10474]. A CRW-based effective pair potential corresponds to the interaction free energy between the two atom groups of an atomistic parent model that represent the coarse-grained interaction sites. Since the CRW coarse-graining procedure does not involve any form of parameterisation, thermodynamic and structural properties of the condensed phase are predictions of the model. We show in this work that CRW-based CG models of polymer-additive systems are capable of predicting the correct structural correlations in the mixture. Furthermore, the excess chemical potentials of the additives obtained with the CRW-based CG models and the united-atom parent models are in satisfactory agreement and the CRW-based CG models show a good temperature transferability. The temperature transferability of the model is discussed by analysing the entropic and enthalpic contributions to the excess chemical potentials. We find that CRW-based CG models provide good predictions of the excess entropies, while discrepancies are observed in the excess enthalpies. Overall, we show that the CRW CG potentials are suitable to model structural and thermodynamic properties of polymer-penetrant systems.

Conditional Reversible Work Coarse-Grained Models with Explicit Electrostatics-An Application to Butylmethylimidazolium Ionic Liquids.

Parametrization of nonbonded interactions is fundamental in the development of new coarse-grained (CG) models. The conditional reversible work (CRW) method is unique among the systematic coarse-graining methods since it requires only the simulation of two molecules in vacuo, making parametrizations of new models computationally inexpensive. This method has been applied successfully to apolar systems and has recently been extended for the parametrization of models with explicit electrostatics. In this work, the ECRW method is used to parametrize models for butylmethylimidazolium (BMIM) ionic liquids with three different anions. Results of subsequent CG simulations are used to illustrate the strengths and weaknesses of the resulting models. In particular, the structural and dynamical properties of the bulk, as well as the behavior at the liquid-vapor interface, are studied. It is shown that the ECRW coarse-graining method produces CG models of high quality for all selected test systems.

Phase Equilibria Modeling with Systematically Coarse-Grained Models-A Comparative Study on State Point Transferability.

Coarse-grained (CG) models for soft matter systems can be parameterized using a variety of approaches. In systematic coarse-graining procedures, effective interactions are calculated using a reference fine-grained simulation. The degree to which thermodynamical and structural properties of the reference system are reproduced depends critically on the coarse-graining method used to parameterize the model. In this article, a comparative study is presented on the capability of different CG models to reproduce vapor-liquid equilibrium thermodynamics of hexane and perfluorohexane systems. CG models are developed using coarse-graining methods based on the reproduction of (a) structure and (b) coupling free energies. Although the structure-based models show an overestimation of the pressure and therefore perform relatively poorly in reproducing the vapor-liquid thermodynamics, it is shown that methods based on coupling free energies, in particular the conditional reversible work method, are capable of better reproducing the vapor-liquid phase diagram of the united-atom reference simulations, while reproducing the liquid structure almost as accurately as the structure-based CG model. This illustrates the state point transferability of the interaction potentials calculated using these methods, as the resulting CG model (which has been parameterized at 300 K) is capable of accurately modeling the thermodynamic properties up to the critical point.

Conditional Reversible Work Coarse-Grained Models of Molecular Liquids with Coulomb Electrostatics - A Proof of Concept Study on Weakly Polar Organic Molecules.

Scale bridging simulations of soft matter rely on the availability of transferable coarse-grained models. In systematic coarse-graining approaches, core principles of statistical mechanics are used to relate the coarse-grained models to the underlying molecular interactions. The conditional reversible work (CRW) method provides effective, nonbonded pair potentials by means of computing coupling free energies between mapped chemical groups. This method has so far been used almost exclusively for systems composed of apolar organic molecules, but additional challenges arise when developing coarse-grained models for polar molecules in which (long-range) electrostatic interactions are important. Herein, we present a modified formulation of the CRW method where we divide the effective interaction potential into van der Waals and electrostatic components. The shape of the effective electrostatic interaction justifies modeling the electrostatics using a Coulomb potential with point charges on each site that are equal to the net charge of the underlying group of atoms. We perform CRW calculations using two polar molecules as test cases (an ether (1,2-dimethoxyethane) and an ester (ethyl propionate)). The results of subsequent liquid state simulations indicate that the coarse-grained models obtained by the new method are of similar quality with respect to representability and thermodynamic transferability as formerly developed models for apolar systems.