Hello, Frequent MD simulator, first time parameterizer. I'm interested in validating my parameters for the organic cation NMDG+ for OPLS/AA. The molecule itself isn't that complicated ( http://www.chemspider.com/Chemical-Structure.8249.html) and I'm quite confident that all the atom types are in OPLS/AA. I've made models using the standard OPLS charges for each atom type and several point charge assignment protocols with Gaussian/RESP consistent with OPLS. I've done explicit water simulations with those in Gromacs, which look fine, but now I'm stumped about how to verify my parameters.
Experimental heats of vaporization and density seem to be common metrics of validation for force field parameters. OPLS parameters were tested against those properties for various ionic liquids (1) and again for a large number of neutral molecules (2). Unfortunately, I have no experimental data for an ionic liquid containing NMDG+. Validating my parameters with only theoretical methods "initially" seemed like a good choice. I calculated the free energy of solvation for my molecule in Gaussian, with implicit solvent, but as I understand it, it's either not possible or very difficult (with a boundary correction, 3) to do free energy perturbation with a charged molecule. This is a topic which is frequently addressed by David Mobley on this mailing list: http://lists.gromacs.org/pipermail/gmx-users/2006-March/020672.html http://lists.gromacs.org/pipermail/gmx-users/2006-September/023942.html http://lists.gromacs.org/pipermail/gmx-users/2009-December/047455.html http://lists.gromacs.org/pipermail/gmx-users/2011-August/063843.html Would the best solution be to contact an experimentalist who studies ionic liquids containing my cation (4) to request experimental data or is there something I'm missing? -Christopher Ing University of Toronto 1. Sambasivarao, S.V., Acevedo, O.: Development of OPLS-AA Force Field Parameters for 68 Unique Ionic Liquids. J. Chem. Theory Comput. 5, 1038–1050 (2009). 2. Caleman, C., van Maaren, P.J., Hong, M., Hub, J.S., Costa, L.T., van der Spoel, D.: Force Field Benchmark of Organic Liquids: Density, Enthalpy of Vaporization, Heat Capacities, Surface Tension, Isothermal Compressibility, Volumetric Expansion Coefficient, and Dielectric Constant. J. Chem. Theory Comput. 8, 61–74 (2012). 3. Kastenholz, M.A., Hünenberger, P.H.: Computation of methodology-independent ionic solvation free energies from molecular simulations. II. The hydration free energy of the sodium cation. J. Chem. Phys. 124, 224501 (2006). 4. Joshi, M.D., Chalumot, G., Kim, Y.-W., Anderson, J.L.: Synthesis of glucaminium-based ionic liquids and their application in the removal of boron from water. Chem. Commun. 48, 1410 (2012). -- gmx-users mailing list [email protected] http://lists.gromacs.org/mailman/listinfo/gmx-users * Please search the archive at http://www.gromacs.org/Support/Mailing_Lists/Search before posting! * Please don't post (un)subscribe requests to the list. Use the www interface or send it to [email protected]. * Can't post? Read http://www.gromacs.org/Support/Mailing_Lists
