The full results are provided in the ESI † as well as Ref. 28. For the sake of clarity and ease of interpretation, most of our discussion will be based on general statistics and we will avoid as much as possible to focus on single instances. Often times, we will com- pare the results to a null hypothesis, which is constructed by taking the average of the training systems shifts. This requires no computation and provides a single estimate of 99.7 cm -1 for all test systems. The RMSD of the null hypothesis for the measured test values is 62.3 cm -1 , which we will take as a general quality measure.