Workshop on Predicting the Thermophysical Properties of Fluids by Molecular Simulation

Ilja Siepmann, Sohail Murad, Daniel Friend, Raymond Mountain, Marc Nyden, Russell Johnson, Bruce Garrett, Fiona Case, David J. Frurip, Jonathan Moore, Joey Storer, Anne Chaka, and Joseph Golab

The development and implementation of theoretical model chemistries, consisting of a series of well-defined quantum mechanical calculations, have led to dramatic improvements in the accuracy of calculated values for the thermodynamic properties of small, gas-phase molecules. Unfortunately, these methods are not now, and may never be, applicable to large, condensed phase systems, such as polymer solutions and blends, because the computational effort required for their implementation increases as a large power of the number of constituent atoms (in some cases exceeding N7); becoming prohibitive even on state-of-the-art supercomputers after about 10 heavy atoms. On the other hand, simulation methods that make use of force fields, parameterized on the basis of quantum mechanical calculations and/or experimental measurements, offer an immediate and practical alternative for the prediction of the properties of molecular fluids, including mixtures. A characteristic common to all simulation studies is that the results are ultimately determined by the accuracy of the force field and the extent to which the relevant phase space is sampled in the calculation. Consequently, extensive validation and testing are needed to ensure that the results are accurate (with respect to their agreement with experimental values for the property of interest) and reproducible (in the sense that that they represent converged values and are not artifacts of inadequate sampling or contrived force field parameters). In the absence of these evaluations, simulation methods can produce variable results, which adversely affect their credibility and limit their use in industry. In response to this need, we propose to launch a program of research and knowledge dissemination to accomplish the following goals.

• The definition of standard problems with agreed upon answers determined by state-of-the-art simulations using available software and which can be used as benchmarks by commercial and academic software developers.
• The development of a set of rules to determine force field parameters for new molecules (such as rules using constituent groups).
• The development and validation of force fields for key compounds (industrially important materials such as: benzene, phenol, glycol, and commodity polymers) and mixtures that are optimized for a wide variety of properties.
• The development of standardized simulation and correlation methods to determine key thermophysical properties (such as, compressibility, solubility, viscosity, phase equilibria, heat capacity, and thermal conductivity).

As part of this program we will also offer tutorials on cutting edge methods, hold open competitions where method developers and practitioners will be encouraged to present results obtained from simulations on a predetermined set of properties and molecular fluids, and work with commercial and academic software developers to make simulation methodologies more widely available.

The purpose of this document is to announce our intention to launch this research and development effort by holding a workshop to highlight and stimulate routes to improving the current state of the art in molecular simulation methods, force field development and validation. This workshop will be held on June 18 and 19, 2001 at the National Institute of Standards and Technology (NIST) in Gaithersburg, MD. The sponsors, in addition to NIST, are the Dow Chemical Company, BP Chemicals, Colgate Palmolive, Pacific Northwest National Laboratory, the University of Illinois-Chicago and the University of Minnesota. The format of the workshop will consist of presentations in which speakers from industry, government laboratories, and academia will identify the properties and compounds of interest and explain the methodologies and force fields that can be used in evaluating these properties. Following the presentations, the workshop participants will organize into breakout sessions to make specific recommendations as to the most appropriate methods and force fields for each of the properties of interest. These recommendations will be published via a website and will be amended to reflect the results of the first open competition, which will be held at a date and time to be determined at the workshop.