where 𝑓i is the force of atom i, 𝐷 ! ! denotes the corresponding model deviation, and l is a constant (1 eV/Å in this study) used to make sure that an atom having a small absolute model deviation will also have a small relative model deviation. For each atom, we can build one molecular cluster, thus a large number of molecular clusters will be extracted from the trajectory in each iteration. Based on the value of the relative force model deviation of the central atom, these clusters can be divided in to three groups named "accurate" (𝐸 ! ! is less than 20%), "candidate" (𝐸 ! ! is between 20 and 45%), and "failed" (𝐸 ! ! is greater than 45%).