Systematic Molecularity-Dependent Entropy Errors in Continuum/RRHO Solution Thermochemistry: Origin and Correction

provides a simple diagnostic case: the electronic association is favorable, and the differential continuum solvation term is small, yet the final 1 M association free energy remains positive and essentially gas-phase-like. Water and chloroform clusters further show how this molecularity-dependent RRHO penalty accumulates with increasing association. Confinement-based corrections, including Martin-Pratt density scaling and Benson's free-volume formulation, reduce this artificial destabilization without modifying electronic energies or continuum solvation terms. The main limitation identified here is therefore not the absolute continuum solvation free energy of isolated species but the use of gas-phase RRHO thermochemical corrections to construct reaction free energies in solution. This distinction provides a physical basis for applying condensed-phase translational entropy corrections in association and activation thermochemistry.