The idea that enzymes accelerate their reactions by entropic effects has played a major role in many prominent proposals about the origin of enzyme catalysis. This idea implies that the binding to an enzyme active site freezes the motion of the reacting fragments and eliminates their entropic contributions, (Δ S _cat ^‡ )′, to the activation energy. It is also implied that the binding entropy is equal to the activation entropy, (Δ S _w ^‡ )′, of the corresponding solution reaction. It is, however, difficult to examine this idea by experimental approaches. The present paper defines the entropic proposal in a rigorous way and develops a computer simulation approach that determines (Δ S ^‡ )′. This approach allows us to evaluate the differences between (Δ S ^‡ )′ of an enzymatic reaction and of the corresponding reference reaction in solution. Our approach is used in a study of the entropic contribution to the catalytic reaction of subtilisin. It is found that this contribution is much smaller than previously thought. This result is due to the following: ( i ) Many of the motions that are free in the reactants state of the reference solution reaction are also free at the transition state. ( ii ) The binding to the enzyme does not completely freeze the motion of the reacting fragments so that (Δ S ^‡ )′ in the enzymes is not zero. ( iii ) The binding entropy is not necessarily equal to (Δ S _w ^‡ )′.

We present an efficient algorithm for generating semiglobal potential energy surfaces of reactive systems. The method takes as input molecular mechanics force fields for reactants and products and a quadratic expansion of the potential energy surface around a small number of geometries whose locations are determined by an iterative process. These Hessian expansions might come, for example, from ab initio electronic structure calculations, density functional theory, or semiempirical molecular orbital theory. A $\text{2×2}$ electronic diabatic Hamiltonian matrix is constructed from these data such that, by construction, the lowest eigenvalue of this matrix provides a semiglobal approximation to the lowest electronically adiabatic potential energy surface. The theory is illustrated and tested by applications to rate constant calculations for three gas-phase test reactions, namely, the isomerization of 1,3-cis-pentadiene, ${\mathrm{OH+CH}}_{\mathrm{4}}\to {\mathrm{H}}_{\mathrm{2}}{\mathrm{O+CH}}_{\mathrm{3}},$ and ${\mathrm{CH}}_{\mathrm{2}}{\mathrm{Cl+CH}}_{\mathrm{3}}\mathrm{F}\to {\mathrm{CH}}_{\mathrm{3}}{\mathrm{Cl+CH}}_{\mathrm{2}}\mathrm{F}.$