I'm sorry for the delay with my reply.
>If eq. 4 from Klamt and colleagues is arguable, what is an alternative way to calculate free energy in solution (with entropy contribution of a solute etc…)?
>In the discussion you mentioned in your answer, you proposed to compare electronic energies, but for comparison of a calculation with experimentally observable values (at least with a distribution between different states in the solution) one needs to take into consideration entropy contributions too.
I'd suggest you to use eq. 4 from Klamt and coworkers
with the only difference that ΔGcorr_gas should include
various gas phase vibrational contributions only.
In addition, you should add the following to your input:
$pcm TABS=the absolute temperature. ICAV=1 IDISP=1 $end
to compute Gnes of eq. 4.
Alternatively, you can experiment with
$pcm TABS=the absolute temperature. ICAV=1 IREP=1 IDP=1 $end
See Firefly's manuals for more details on PCM input.