MGCDB84 is compiled from the work of numerous groups. It consists of 84 datasets with 4986 data-points, which includes a number of chemical properties. All but two datasets are arranged into 8 subsets based on the types of chemical reactions involved, these are summarised in table 1. The other datasets consist of absolute atomic energies (AE18) and rare-gas dimer potential energy curves (RG10). The full database is split into three sets for training, validation, and testing, with datasets from each subset distributed among these. We use the training set to fit the functional parameters; it contains 25 datasets and 870 data-points. We use the validation set to determine the whether the fitted functional is transferable; it consists of 35 datasets with 2964 data points. We only use the test set to calculate the accuracy of the final functional; it contains 24 datasets with 1152 data-points. This same splitting was previously used to fit the ωB97M-V and ωB97M(2) functionals. The functional ωB97X-osUW12-D3(BJ) consists of multiple parameters; the linear parameters are selected and optimized using the procedure outlined below, while the non-linear parameters are fixed in advance of the optimization. The range-separation parameter ω = 0.3 was chosen as it is the value used in the ωB97X and ωB97X-V functional. The B97 parameters γ x , γ c,ss , γ c,os remain fixed using the original B97 values. The non-linear dispersion parameters were chosen as a 1 = 0.00 and a 2 = 5.49. These were taken from the ωB97X-D3(BJ) functional, a re-implementation of ωB97X-V with a D3(BJ) correction instead of the non-local VV10 component. In addition these parameters are almost identical to those optimized for our dispersion-corrected B-LYP-osUW12-D3(BJ) functional, for which a 1 = 0.0 and a 2 = 5.45. Unlike the other non-linear parameters, the UW12 Gaussian exponents are not chosen to reproduce an observable effect per-se. Instead they are chosen to give a degree of flexibility in the final fit of the geminal function. To give sufficient flexibility, the exponents were chosen to be the set {γ -n , . . . , γ n } with n = 4 and γ = 3.16228. This covers a number of orders of magnitude for the scale of the geminal.