The real parameter λ in Eq. ( ) is generally not optimized but set as a universal constant for all internuclear distances. Note that the magnitude of λ is related to the potential changing along the distances between fragments in the reactant or product asymptotes. For neutral molecular systems without long-range interactions, λ is generally set as a number around 1.0 Å. For systems with longrange interactions, such as the ion-molecule reaction, a large number should be adopted for λ. The symmetrization operator, Ŝ , in Eq. ( ) contains all permutation operations between like atoms associated with the molecular system. For the F + H2O system, In Eq. ( ), the number of the input PIPs is denoted as I. The number of neurons in the first and second hidden layers are denoted as J and L, respectively. The hyperbolic tangent function, 2 2 tanh( ) ( ) / ( )