Given that the training data was available for all the groups, any participant who also computed the training set (even if partially) would have been in a position to provide uncertainty estimates based on the performance of the method. Learning strategies heavily dependent on the latter would likely underestimate the uncertainty, but could provide this information nonetheless. LS1 provided a rather small uncertainty of 8 cm -1 , similar to that of LS4 (7 cm -1 ). Both made use of linear fitting expressions, with comparable fitting quality for the test set. In the case of FA1, the only other submission with uncertainty quantification, the value was 16 cm -1 , taken to be slightly above the mean average error of the training set quasi-classical simulation results.