Hence, the total reduction in the activation barrier can be written as some function related to the value of ϕ m,n , the strain in that mode µ n and the force constant of the mode, κ n : ∆E a = n f n (ϕ n , µ n , κ n ). Under the harmonic approximation, those functions would become 2 hω n µ 2 ). This results is equations reminiscent of the simpler formulation of Bell, 49 where µ corresponds to the reaction coordinate displacement x, and 1 2 hω n µ 2 ) is the force required to achieve that displacement in a harmonic system.