Here, h = θ (x m -x ‡ m ), defined as a Heaviside step function, is an operator projecting the wave packet onto the right side of a dividing surface x ‡ m . For a single molecule with the symmetric double well potential as described in Sec. II A, we assume that the reactant and product region are separated by the dividing surface at x ‡ m = 0. The reactant population in the left well region is thus computed as