Objective

Test the transferability of a force field between similar molecules for a given property.

Challenge

For the following molecules:

(i)   1,4-butanediol
(ii)  1,3-butanediol
(iii) 1,2-butanediol
(iv) 2-methyl-1,3-propanediol
(v)  1,2,4-butanetriol

Compute the low-strain rate limit (Newtonian) viscosity at the following state points:

(a) T = 373K, P=0.1MPa
(b) T = 373K, P=250MPa



Rules of the game

• The force field employed must be capable of describing the interactions of each of the molecules in a consistent manner. The same force field must be used in all calculations.

• Any force field previously published in the open literature prior to the announcement of this challenge is acceptable, as long as its development adheres to the condition above.

• Force fields may be parameterized using any physical property data (including viscosity) describing alkanes or alcohols with the following exception: no data from diol, triol, or “n-ol” species may be used in the force field development.  This rule does not apply to force fields published in the open literature prior to the announcement of the challenge.

• Although the state points are specified at constant pressure, there is no requirement to use constant pressure sampling for calculations. However, if a constant density is used as an input as part of the calculation, the entrant must demonstrate that the density selected is consistent with the pressure of the target property estimation. In other words, the density must be consistent with equation of state of the model employed.

• Estimates of uncertainty for computed viscosities must be included.

• Other methods – group contribution methods are acceptable provided that the spirit of the above rules are followed with respect to parameterization and application.  Compliance will be determined on a case by case basis.

Contest Scoring

• Quantitative Accuracy Metrics (50%)
  
  1. At each state point, full credit will be awarded for predictions within the uncertainty limit of 5% of the experimental value.
  
  2. A linear interpolation of partial credit will be awarded for predictions with an absolute deviation above the minimum threshold and a maximum of 40%. No points will be awarded for prediction above the maximum deviation.

• Relative Accuracy Metrics (50%) There are two categories associated with relative measures of model performance, with scoring split equally between the two.
  
  Relative Ranking (25%)
  
  1. For the predictions at the low pressure state point (373K and 0.1MPa), the computed viscosities of each species will be compared to the experimental values. The molecule that is in best agreement to experiment will be used as a normalization of the remaining values to determine relative viscosities. The relative ratios will be compared to the corresponding normalized ratios using the experimental data. The quantity (eta/etaref)simulation will receive full credit if it is within 5% of the corresponding quantity (eta/etaref)expt . As in the quantitative accuracy section, similar partial credit will be awarded on a sliding scale.
  
  Relative Viscosity increase (25%)
  
  2. The relative rise in viscosity with pressure from simulation will be compared to the corresponding experimental values by computing the quantity (eta[250MPa]/eta[0.1MPa]). Full credit is awarded if the simulation and experimental quantities are within 5% of each other; similar partial credit will be awarded on a sliding scale.

• Molecule Problem Scoring Spreadsheet (MS Excel file)

Other entry guidelines

• A submission for this challenge problem is to be in the form of a manuscript suitable for submission to a refereed, archival, scientific journal. The manuscript must contain sufficient detail about the simulation method and about the force field so that an experienced simulator could reproduce the results without requiring access to proprietary information. In particular, all potential parameters and molecule geometry parameters must be explicitly specified in the manuscript. The results are to be reported in SI units. A randomly selected subset of the submitted predictions will be validated by the judges by reproducing the reported calculations.

• An analysis of the uncertainty in the calculated results is required and must be included in the manuscript.

• Entries are expected to present results that are statistically significant and to present sufficient supporting evidence to establish this quality. Also, the scientific reasoning behind any new (unpublished) force field parameterizations must be clearly spelled out in the entry. If there is a consensus among the judges that an entry is of poor quality (uses a method commonly accepted to be fundamentally flawed, presents results that are not statistically significant, fails to provide sufficient supporting data and details, violates the various rules and guidelines established for the competition, or for any other reason would be unlikely to be accepted by any peer-reviewed scientific journal in the field), that entry will be rejected and will not be considered in the judging.

• Entries that represent collaborations between multiple research groups are welcomed.

Background

• Viscosity is a fundamental transport property of primary importance in lubrication, having a great impact on heat transfer, friction and wear characteristics, and energy efficiency of lubricated contacts.

• Fluids entrained in lubricated contacts of non-conforming surfaces typically are compressed to very high pressures, in the range of 1 GPa or more. Experimental data describing viscosity at such pressures are scarce, and are not easily predicted on the basis of viscosity data at atmospheric pressure.

• While real lubricants are typically of larger molecular weight than the small molecules considered in this challenge, a force field that successfully demonstrates the properties of transferability demanded in this contest challenge could be used to predict the properties of larger diol and triols of interest as potential lubricant basestock components.