In this trade-off for the sake of simplicity does reduce the number of parameters needed. As an upper bound, the number of parameters grows quadratically with the number of elements considered (the scaling is 3(N + 1) 2 , where N is the number of elements, and the factor of 3 comes from the number of typical bond types in a molecule i.e., single, double, and triple), though the number of parameters needed is less, as not all bond orders are valid between all pairs of elements.