# CCL: COSMO vs COSMO-RS

*From*: Andreas Klamt <klamt-#-cosmologic.de>
*Subject*: CCL: COSMO vs COSMO-RS
*Date*: Thu, 31 Jan 2013 13:35:15 +0100

Sent to CCL by: Andreas Klamt [klamt=cosmologic.de]
Dear Sina,

`there are several potential reasons for the large difference. On the
one
``hand you used qute different basis sets for the COSMO calculations at
``eps=78 and the eps=infinity calculations. Admittedly I do not expect
``that this changes the results too much.
`

`I am not completely sure how you did the COSMO-RS reaction
calculation.
``Did you use the thermodynamic cycle inserting the gasphase reaction
``energy from the b3-lyp and a def2-QZVP calculation? If so, then the
``latter result, i.e.
` GReaction-COSMO-RS-Water - GReaction-gas= -2.05 Kcal/mol

`should be nothing else than the dG_solvation difference of educts and
``products on BP-TZVP-COSMO-RS level, since GReaction-gas cancels out.
` Can you just check and confirm that?

`Please note that bare COSMO (unlike other implicit solvation models)
is
``not parameterized for giving optimal dG-solvation values. It just trets
``the electrostatic component, and not the dispersion, and other
``non-electrosttic contributions. Hence you cannot expect good
``quantitative results for dG_hydrtion from using bare COSMO. COSMO-RS is
``my recommended method to take all these things into account.
`

`Finally it should be noted that the recommended procedure for
``dG_reaction calculation requires to include the vibr., rot. and transl.
``free energy contributions in the gasphase, which will be very important
``for a reaction in which the number of particles changes from 1 to 4.
``These can be calculated for exmple with the freeH tool in TURBOMOLE.
But
``this contribution would appear in the gasphase as well as in the liquid
``phase and thus should not influence your differential results.
`

`In summary, I do not find your result too surprising. The more relible
``answer should be the COSMO-RS answer, but please note that your
``calculations are still suboptimal.
` Best regards
Andreas
Am 31.01.2013 00:13, schrieb Sina Behtash behtash!=!email.sc.edu:

Sent to CCL by: "Sina Behtash" [behtash ~ email.sc.edu]
I am using both COSMO (implemented in Turbomole 6.0) and COSMOtherm/COSMO-RS to
calculate the solvent effects on the free energy of reaction for the following
reaction:
Glucose --> HMF + 3 H2O
First, I optimized all the reactants and products in the gas phase using b3-lyp
and a def2-QZVP basis set. Next, assuming that the geometry does not change
significantly in the presence of water, I used the optimized structure to run a
COSMO single-point energy calculation with the same basis set and dielectric
constant of water at 298 K which is 78.39. According to the COSMO calculation,
GReaction-COSMO - GReaction-gas= -15.2 Kcal/mol
Next, in order to generate relevant input for COSMO-RS calculations, I ran
another COSMO single-point energy calculation using the BP functional and TZVP
basis set. Then I used COSMOtherm/Reactions and the BP-TZVP data base to run
COSMO-RS calculations for my reaction at T=298K.
The result shows that,
GReaction-COSMO-RS-Water - GReaction-gas= -2.05 Kcal/mol
I was expecting that for water, the COSMO and COSMO-RS calculations should give
almost the same result. However, in this case the difference in the effect of
water on the free energy of reaction is around 13 Kcal/mol. Can anyone explain
to me why the difference is so large? Im wondering if the effects of hydrogen
bonding and electrostatic interactions can be this significant.
Sina Behtash
Ph.D. Student
Department of Chemical Engineering
University of South Carolina
Phone: (803)777-0611
Email: behtash-$-email.sc.edu>

--
Prof. Dr. Andreas Klamt
CEO / Geschäftsführer
COSMOlogic GmbH & Co. KG
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phone +49-2171-731681
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[University address: Inst. of Physical and
Theoretical Chemistry, University of Regensburg]
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