On the relationship between free energy difference calculations by thermodynamic integration and harmonic frequency analysis

Harmonic frequency analysis (HFA), based on statistical mechanics, is a widely used and powerful tool for evaluating free energy changes between molecular states. It has, as such, been employed extensively to evaluate the free energy of reaction and activation for chemical processes. Alternatively, free energy differences can be calculated using thermodynamic integration (TI). In TI, the force on a constrained reaction coordinate is calculated, and this force from a to b is integrated to obtain the Helmholtz free energy change ΔAab. Although HFA and TI clearly are related from a fundamental statistical mechanics point-of-view, the relationship is not immediately obvious when one considers the quite different procedures applied in the two methods. This article provides a detailed analysis and proof of the relation between HFA and TI.