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As with the initial alcohol-ester study, the objective function was observed to rapidly decrease after the first iteration, after which there was only marginal improvements (Figure 8).
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Figure 8. The value of the objective function at each iteration for each refit performed.
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Figure 9. The average RMSE of each type of property at each iteration relative to the RMSE of each type of property estimated using the initial force field parameters.
In general most of the refit parameters changed only slightly from their initial values, varying less than 5% in most cases (Figure 10)
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Figure 10. The change in the each parameter for each of the different training sets relative to their starting value (taken from the openff-1.0.0 force field).
The gradient of each properties contribution to the objective function during the first and second iterations were extracted and are displayed in Figure 11. In general, each property contributes largely appears to constrain the different parameters in the same direction, with the exception of XXX.
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b)
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Figure 11. The contribution of each type of property to the gradient of the objective function at iteration 0 and 1when refitting against a) enthalpy of mixing and binary mass density and b) pure density and enthalpy of vaporisation. Solid lines represent the true direction and magnitude of each gradient, while the dashed lines represent the same gradients but which have been normalised to enhance clarity when the magnitude of a gradient is small.
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