Chunxia Chen

A comparison of united atom, explicit atom, and coarse-grained simulation models for poly(ethylene oxide).

We compare static and dynamic properties obtained from three levels of modeling for molecular dynamics simulation of poly(ethylene oxide) (PEO). Neutron scattering data are used as a test of each models accuracy. The three simulation models are an explicit atom (EA) model (all the hydrogens are taken into account explicitly), a united atom (UA) model (CH(2) and CH(3) groups are considered as a single unit), and a coarse-grained (CG) model (six united atoms are taken as one bead). All three models accurately describe the PEO static structure factor as measured by neutron diffraction. Dynamics are assessed by comparison to neutron time of flight data, which follow self-motion of protons. Hydrogen atom motion from the EA model and carbon/oxygen atom motion from the UA model closely follow the experimental hydrogen motion, while hydrogen atoms reinserted in the UA model are too fast. The EA and UA models provide a good description of the orientation properties of C-H vectors measured by nuclear magnetic resonance experiments. Although dynamic observables in the CG model are in excellent agreement with their united atom counterparts, they cannot be compared to neutron data because the time after which the CG model is valid is greater than the neutron decay times.

Comparison of explicit atom, united atom, and coarse-grained simulations of poly(methyl methacrylate).

We evaluate explicit atom, united atom, and coarse-grained force fields for molecular dynamics simulation of poly(methyl methacrylate) (PMMA) by comparison to structural and dynamic neutron scattering data. The coarse-grained force field is assigned based on output of the united atom simulation, for which we use an existing force field. The atomic structure of PMMA requires the use of two types of coarse-grained beads, one representing the backbone part of the repeat unit and the other representing the side group. The explicit atom description more closely resembles dynamic experimental data than the united atom description, although the latter provides a reasonable approximation. The coarse-grained description provides structural and dynamic properties in agreement with the united atom description on which it is based, while allowing extension of the time trajectory of the simulation.

Why are coarse-grained force fields too fast? A look at dynamics of four coarse-grained polymers

Coarse-grained models decrease the number of force sites and thus reduce computational requirements for molecular simulation. While these models are successful in describing structural properties, dynamic evolution is faster than the corresponding atomistic simulations or experiments. We consider coarse-grained models for four polymers and one polymer mixture, where accurate dynamics are obtained by scaling to match the mean-squared displacements of the corresponding atomistic descriptions. We show that the required scaling is dictated by local friction and that this scaling is only valid after the onset of continuous motion.

Piezo‐ and elasto‐optic properties of liquids under high pressure. III. Results on twelve more liquids

Precision optical interferometric measurements at high pressures have been made on twelve liquids. The liquids studied are chloroform, carbon disulfide, o‐xylene, m‐xylene, 1‐3‐5‐trimethyl benzene, cumene, methyl cyclohexane, ethyl bromide, ethyl iodide, isobutyl alcohol, ethyl acohol, and deuterium oxide. The piezo‐ and elasto‐optic properties of these liquids are found to be similar to those of other liquids described in Papers I and II of this series. For all these liquids, the data reveal pronounced nonlinear piezo‐optical (Δn vs P) behavior, but linear elasto‐optical (Δn vs e, when e is the Eulerian strain) relationship. Thus, the Eulerian representation of strain is advantageous in describing the elasto‐optic behavior of liquids, irrespective of the shape, size, and composition of the molecules of the liquid. The linear relationship between Δn and Eulerian strain can be used to discriminate between the various equations of state. It may also provide a simple empirical expression for the computation ...

Importance of using Eulerian representation of strain in high pressure studies on liquids

It is shown that all the published data on the static dielectric constant of liquids at high pressures can be expressed in simple linear relationship between √e0 and e, the Eulerian strain, just like earlier reported relation between √e∞ and e. The reason for the necessity of using the Eulerian representation in such high pressure studies is presented.

Molecular calculation of the dielectric constant of N2 and CO2

The formal equation which relates the deviation from the Clausius–Mossotti equation for the dielectric constant to the dipoles induced in a nonpolar molecular fluid by the external field‐induced dipoles and the permanent multipole moments is taken as a starting point. The molecular integrals appearing in this equation are explicitly written in terms of angle‐averaged and angle‐dependent pair correlation functions for the fluid. The intermolecular interactions for N2 and CO2 are modeled as the sums of diatomic potential functions plus quadrupolar energies. Simulated pair correlation functions are used in a calculation of the two‐ and three‐molecule contributions to the deviation from Clausius–Mossotti behavior for liquid N2. Pair correlation functions calculated in the limit of zero density were used to evaluate second dielectric virial coefficients for N2 at 300 and 124 °K and for CO2 at 323 °K. A comparison with experiment is moderately satisfactory, considering the uncertainties in the parameters of the...

Piezo‐ and elasto‐optic properties of deuterium oxide under high pressure

The variation of the refractive index of D20 with pressure has been determined using g the high precision optical interferometric method.(AIP)