where l is the number of terms, r ij is the distance between atoms a i and a j , T k (a i , a j , r ij ) is the k-th term and its weight is w k . If r ij exceeds the given value r max , the terms for this pair of atoms are set to zero. In Vinardo, r max is equal to 64 Å10 , and the following terms and weights are used: