  The entropies of molecules in solution are routinely calculated using gas
phase formulae. It is assumed that, because implicit solvation models are
fitted to reproduce free energies, this is sufficient for modeling reactions in
solution. However, this procedure exaggerates entropic effects in processes
that change molecularity. Here, computationally efficient (i.e., having similar
cost as gas phase entropy calculations) approximations for determining
solvation entropy are proposed to address this issue.The $S_\omega$,
$S_\epsilon$, and $S_{\epsilon\alpha}$ models are nonempirical and rely only on
physical arguments and elementary properties of the medium (e.g., density and
relative permittivity). For all three methods, average errors as compared to
experiment are within chemical accuracy for 110 solvation entropies, 11
activation entropies in solution, and 32 vaporization enthalpies. The models
also make predictions regarding microscopic and bulk properties of liquids
which prove to be accurate. These results imply that $\Delta H_\text{sol}$ and
$\Delta S_\text{sol}$ can be described separately and with less reliance on
parametrization by a combination of the methods presented here with existing,
reparametrized implicit solvation models.
