where is the number of types of properties (e.g. density, enthalpy of vaporization, etc.), is the number of data points of type , is the experimental value of data point and ( ) is the estimated value of data point using the current force field parameters. The denominator is an inverse weight with the same units as property type chosen so that that each property type contributed approximately equally to the objective function. For example, for the pure training set, ∼ 50% of the objective function value is due to data, and ∼ 50% is due to Δ . This a priori approximation was made as it is unclear that any one type of property should be weighted more than another.