where r eq and θ eq are equilibrium structural parameters ; K r , K θ , V n are so called "force constants"; n is multiplicity and γ is phase angle for torsional angle parameters. The ϵ, σ, and q parameters characterize the non-bonded vdW potential. ϵ, σ follow Lorentz-Berthelot combination rules. The GAFF parameters {K r , r eq , K θ , θ eq , V n , ϵ, σ} are directly read from parameters table according to corresponding atom types while n = 1, 2, 3, 4 and γ = 0 or π.