Is there any keyword missing to get an idea of the dG solvation free energy of a molecule?
# M062X/6-31G* Opt SCRF=(SMD,1stVac,Solvent=water)
From what I understand from the gaussian09 user manual the keyword DoVacuum will do both a calculation in vacuum and with water as a solvent and will calculate the output accordingly i.e. I don't have to substract the gas phase calculation from the result I get with the SMD calculation, right?
From the gaussian16 manual I get that DoVacuum=1stVac, and DoVacuum is deprecated
The result I'm looking for will be after the last optimization in the line that goes (SMD-CDS) ....., correct?
Related question: For a hydroxyl group I would expect something around 4-5 kcal/mol solvation free energy. However using M062X/6-31G* (or M062X/cc-pVTZ) I get only a minor change in a series of similar molecules with and without OH-group.
eg. using # M062X/6-31G* Opt SCRF=(SMD,1stVac,Solvent=water) I get:
hexan.out: SMD-CDS (non-electrostatic) energy (kcal/mol) = 3.47
hexanol.out: SMD-CDS (non-electrostatic) energy (kcal/mol) = 3.57
I'd expected hexanol to be < 0 kcal/mol
Thanks in advance
Christoph Sager
You should not trust the crude (dielectric continuum) model solvation free
energies too much.They ignore completely the chemical structure of the solvent and its structural organization around complex solutes (entropic terms in the free energy balance).The whole problem of solvation free energies is unsolved both classicaly and quantum mechanically ,but there are better approximations than the primitive models packed in the black box packages
"...but there are better approximations than the primitive models packed in the black box packages"
Could you give an example, please?
Consult Schroedinger company for some available methods.My own advanced methods have not been casted in a user friendly form yet due to lack of funding and manpower.
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