Hello everyone

my Name is David and as part of my master thesis I want to calculate diffusion coefficients for n-alkanes (in the range of roughly 10- C) and aromatic hydrocarbons. (like 2-Ethylnaphthalene, Phenanthracen etc.) in water

I decided to use the OPLS-all atom forcefield and the TIP4P/2005 water model. I already did a lot of measurement and found good agreement for the Diffusion coefficient in water for heavier molecules like ethlnaphthalene, bisphenol a. For alkanes, I found a good correlation for heavier atoms with the model proposed by Oelkers (Calculation of diffusion coefficients for aqueous organic species at temperatures from 0 to 350 °C; 1991). Unfortunately, there seems to be no Data for long alkanes in water so these model is my only reference right now.

However, for smaller molecules the measured Diffusion coefficient is around 50-60% off the experimental values (e.g. benzen,methane,ethane). So, my question: Is this a common problem for OPLS-AA? Is there some possibility to improve the Diffusion coefficent? Any suggestions or comments are appreciated

I already tried to increase the box size so one solute is in a box with 3400 water molecules. MSD of 20 runs of the same system are averaged to calculate the Diffusion coefficent.

i attached the used .itps for benzene and tip4p/2005 aswell as my mdp files for equilibration and production run.

Thanks in advance and best regards

David, Master Student

Chair of process systems engineering

Gregor-Mendel-Straße 4

85354 Freising

Germany