where the first term is the potential associated to the length 𝑙 𝑖 of each bond, the second term is the potential of the angles 𝜃 𝑗 and the third term is the potential of the dihedral angles 𝜙 𝑘 . 𝑘 𝑏𝑖 , 𝑘 𝑎𝑗 and 𝑘 𝑑𝑘 are respectively the bond, angle and dihedral force constants, 𝑙 0𝑖 indicates the equilibrium bond lengths, 𝜃 0𝑗 the equilibrium angle values, 𝑛 𝑘 the periodicity and 𝜙 0𝑘 the phase shift of each dihedral.