Following Kramers, we describe the dynamics of an isomerization reactionA⇆Bas a diffusion process in a bistable intramolecular potential. In order to calculate the chemical relaxation timetr, we rely on a purely equilibrium approach, introduced by Chandler, and we focus on the properties of the time correlation functionC(t) = ⟨δNA(0) δNA(t)⟩, and of its derivatives. A plateau formalism permits the extraction of the long relaxation timetrfrom the short time dynamics of a molecule which starts att= 0 from the activated region betweenAand B. The calculation makes it possible to encompass in a unified way the low and high friction limits, which are usually treated differently. It involves the definition of a species specification function σA, which indicates microscopically if a molecule isAorB. Most of the time, σAis taken as a Heaviside function, although its definition should be experiment-dependent. We examine the possibility of taking a wide species specification, presenting a smooth variation in the activated region.