Least popular vibrational entropy model provides the best accuracy and robustness

Abstract

Vibrational contributions into free energies usually amount to several kcal/mol and can significantly affect computational predictions. However, they are generally estimated incorrectly for chemical systems in solutions because the usual (employed in ∼99% of cases) models of vibrational entropies are extremely sensitive to errors in low-lying frequencies (below 300 cm−1), and these low-lying frequencies involve solvent molecules that are usually neglected (computed implicitly) in quantum chemical calculations. We find that only one vibrational entropy approximation—the one proposed by Truhlar in 2011—which is used in only ∼2% of cases, is stable in the low-lying frequency region and does not exhibit this problem. Accordingly, this approximation shows the best accuracy and robustness on a diverse set of experimental complexation energies and can be somewhat improved even further.