Many molecular phenomena are sensitive to energy differences on the order of kT
Conventional organic and biochemistry examine the motions and reactions of molecules in a solvent. When these molecules are large, they are typically flexible, changing shape through rotation around bonds (that is, through conformational degrees of freedom). Multiple reactions or conformations are often possible. Both the rates of reactions and the equilibrium ratios of products vary in proportion to exp(−ΔE/kT). In rate calculations, ΔE is the increase in free energy required to pass through the transition state; in equilibrium calculations, ΔE is the net free energy change after reaching the product state. (As usual, kT is the product of the Boltzmann constant k and the temperature T).
At room temperature, kT is about 4 zJ (0.6 kcal/mole). Differences of 2.3 kT (9.5 zJ, or 1.4 kcal/mole) change rates and equilibria by a factor of 10. Many practical chemical questions involve the rates and equilibria of chemical and conformational changes. Order-of-magnitude errors sharply limit the usefulness of predictions for example, a prediction of 90% yield for a reaction that actually yields 10% could lead to a bad choice of synthetic strategy. Accordingly, claims of “chemical accuracy” for computational models typically imply that errors in computed energies are less than 1 to 2 kcal/mole.