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I'm trying to compute the free energy of solvation of a molecule in gaussian09 using the continuum solvation model, but I am haveing difficulties, I don't know how to do it and gaussian manual does not help me. Someone has any input to me see the comand lines?

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The $\delta G_\text{solv}$ can be computed as $$\delta G_\text{solv}=E_\text{opt in solv} - E_\text{opt in GP}$$ where $E_\text{opt in solv}$ is the energy of the optimised geometry using the scrf=(METHOD,solvent=SOLVENT) keyword, and $E_\text{opt in GP}$ is the energy of the optimised geometry in gas-phase.

This requires the approximation that the thermochemical energy correction (arising from the hessian diagonalization) is equal in both geometries, which can be used for most of the cases.

There are different methods to simulate the electrostatic potential of the solvents: SMD, PCM (in two versions), ALPB (not present neither in G09 nor G16), and GBSA (not present neither in G09 nor G16). To choose among them have a little benchmark to understand which one helps you describe at its best the system solvation energy.

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    $\begingroup$ I would add that the solvation energy often needs to be corrected to change between 1 atm and 1 M concentration. manual.q-chem.com/4.3/sect-solvent.html $\endgroup$
    – S R Maiti
    Jan 19, 2023 at 17:03
  • $\begingroup$ Thank you so much, but let me see if I understand correctly. So I need to do two calculations, one using for exemple the following line #p opt freq MP2/6-31++G** SCRF=(SMD,solvent=acetonitrile) and another calculation like #p opt freq MP2/6-31++G** SCRF=(SMD, dovacuum), right? $\endgroup$
    – AlfredV
    Jan 19, 2023 at 17:30
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    $\begingroup$ the "vaccum" one you can ommit the keyword scrf, so it would be #p opt freq MP2/6-31++G** $\endgroup$ Jan 19, 2023 at 18:32

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