In many cases, variational transition states for a chemical reaction are significantly displaced from a saddle point because of zero‐point and entropic effects that depend on the reaction coordinate. Such displacements are often controlled by the competition between the potential energy along the minimum‐energy reaction path and the energy requirements of one or more vibrational modes whose frequencies show a large variation along the reaction path. In calculating reaction rates from potential‐energy functions we need to take account of these factors and—especially at lower temperatures—to include tunneling contributions, which also depend on the variation of vibrational frequencies along a reaction path. To include these effects requires more information about the activated complex region of the potential‐energy surface than is required for conventional transition‐state theory. In the present article we show how the vibrational and entropic effects of variational transition‐state theory and the effective potentials and effective masses needed to calculate tunneling probabilities can be estimated with a minimum of electronic structure information, thereby allowing their computation at a higher level of theory than would otherwise be possible. As examples, we consider the reactions OH+H₂, CH₃+H₂, and Cl+CH₄ and some of their isotopic analogs. We find for Cl+CH₄→HCl+CH₃ that the reaction rate is greatly enhanced by tunneling under conditions of interest for atmospheric chemistry.