CCL: solvation energy in mopac

 Sent to CCL by: Andreas Klamt [klamt#%#cosmologic.de]
 Dear Daniel,
as author of COSMO in MOPAC I like to emphazise that the COSMO in MOPAC is not parameterized for the quantitative calculation of dG_solv. It is definitely good for getting the calculations closer to solvation than just a gasphase calculation, but for two reasons I recommend higher levels for optimal calculation of dG_solv: 1) The electrostatics of semi-empirical methods is usually not very accurate (because this mostly is not in the target properties of the parameterizations). At least in AM1, PM3, and PM5 (we did not test PM6) dipole moments of some functional groups can be off by one Debye (e.g. nitro groups) and if the dipole moment is of that much the solvation energy will be very bad. This is the reason why I left the semi-empirical level for quantitative solvation calculations a few years after developing COSMO in MOPAC. And most likely this is also why the Minnesota group developed parameterized charge models based on semi-empirical methods in order to improve the SMx models. Since HF is generally overestimating polarities, this is also not extremely reliable. The good message is that DFT is very reliable in that regard, and this quite independent of the special functional. From my perspective DFT is currently the method of choice, although for getting the solvations energies much better than 0.3 kcal/mol one will need even more accurate methods than DFT. By the way, a semi-emirical geometry furnished with a single point DFT calculation can be a very good choice for calculating solvation energies. 2) The second reason is that a dielectric continuum solvation model alone cannot give the complete solvation free energies. One needs in addition "non-electrostatic contributions". While in the SMx and PCM models these have been parameterized with many additional, solute and solvent specific parameters and descriptors, I have never implemented such non-electrostatic terms in MOPAC, at least not quantitatively. MOPAC/COSMO thus does not have these terms. Instead, I have attacked this question with the much more rigorous COSMO-RS approach, which combines COSMO with a statistical thermodynamics of interacting surfaces (using the COSMO polarization charges for the quantification of the interactions) and thus describes solutes and solvents on the same quantum-chemical footing, gives enthalpies and entropies, temperature dependencies, and allows for the treatment of mixtures. But due to the earlier finding of 1) I have only developed COSMO-RS quantitatively on the DFT level. Based on a comparison on ~ 2500 dG_solv predictions (see "On the Performance of Continuum Solvation Methods. A Comment on "Universal Approaches to Solvation Modeling"", Acc. Chem. Res., 2009, 42 (4), pp 489--492) and on the outcome of the recent SAMPL blindtest ( publications in preparation) I dare to say that COSMO-RS currently is the most accurate method for the prediction of dG_solv. 
 Andreas
 Daniel Glossman-Mitnik dglossman%a%gmail.com schrieb:
 
 Dear netters:
How should one calculate the solvation energy of a given molecule using MOPAC 2009? It is enough with doing one calculation in gas phase and another in solvent and then 
 substracting both heats of formation?
 Any example input related to these questions will be appreciated.
 Best regards,
 Daniel
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 Dr. Daniel Glossman-Mitnik:
 Centro de Investigación en Materiales Avanzados, SC
Grupo NANOCOSMOS - Nanotecnología Computacional, Simulación y Modelado Molecular Miguel de Cervantes 120 - Complejo Industrial Chihuahua - Chihuahua, Chih 31109, Mexico 
 Phone: +52 614 4391151 Secretary/FAX: +52 614 4394852 Lab: +52 614 4394805
E-mail: daniel.glossman[A]cimav.edu.mx <mailto:daniel.glossman%5BA%5Dcimav.edu.mx> dglossman[A]gmail.com <mailto:dglossman%5BA%5Dgmail.com> 
 WWW: http://www.cimav.edu.mx/cv/daniel.glossman
 http://blogs.cimav.edu.mx/daniel.glossman
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