Here Xi,QM is the reference data (experiment or QM) and Xi,ReaxFF is the corresponding value of ReaxFF and σ is the accuracy parameter (inverse of training weight) for the i-th data. In the singleparameter search optimization scheme, one force field parameter at a time is varied by a small amount to minimize the overall error (Eq. 1) in the training data. This method is further described by van Duin et al.. , and is implemented in the standalone ReaxFF code, which can be made available on request .