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I need to simulate a salt diluted in a liquid electrolyte. My experience is with LAMMPS, but so far I only had coarse grained toy models, and setting up the force field was straightforward. Now I have a real molecule (>20 atoms), which I optimized quantum mechanically with MP2 and I have tried to run various tools to automatically get the parameters: VMD/topotool, ATB+moltemplate. Nothing really works and I need to tweak too many parameters by hand. So I am wondering whether or not to use an all-atoms MD software such as Amber or Gromacs. Which force field would be the best candidate for my study case considering the physical system? which one is more easy to use when it comes to infer the force field from QM results?

Thank you

Marco

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  • $\begingroup$ Welcome to our site! $\endgroup$ – Camps Apr 1 at 10:31
  • $\begingroup$ Gaff has worked decently for ionic liquids $\endgroup$ – B. Kelly Apr 3 at 6:44
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I would recommend GAFF or OPLS/A

Camps has outlined the OPLS FF, so I won't elaborate very much, but to say that when using LigParGen, a nice feature is that forcefields are made for many different MD programs such as openMM, Gromacs, CHARMM and others, so it is pretty easy. If you email Jorgensen, you can get a version of LigParGen that you can use on your own computer rather than having to use a web browser.

I also recommend the General Amber Force-Field (GAFF). It relies on using a program called Antechamber to make the intermolecular parameters from a lookup table, so geometry optimization does not matter - but, you then either make partial charges from restrained electrostatic potential (RESP) calculations, which does depend on the geometry, or AM1-BCC, which does not depend on the geometry you provide, but to a degree, depends on the conformer you provide. Part of AM1-BCC is that first it optimized geometry using AM1 which is semi-empirical and not all that accurate, but the BCC part is a correction, so they are usually okayish.

Antechamber is easyish to use.

I would avoid doing QM calculations for intermolecular parameters such as bonds, angles, torsions unless you really know what you are doing.

Here is a tutorial on GAFF and Ionic Liquids by David Case, a developer of AMBER

GAFF tutorial for Ionic Liquids

Here are some publications of Ionic Liquids with GAFF

The General AMBER Force Field GAFF can accurately calculate... ionic liquids

Interaction and Dynamics of ionic liquids...

I have also used GAFF for Ionic Liquids, but do not have a publication out yet, but will, one day, I keep telling myself... I can say that it works.

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Which force field would be the best candidate for my study case considering the physical system?

The Optimized Potentials for Liquid Simulations (OPLS) force field can be a possible solution. It was developed with liquids in mind. The main developer is professor William Jorgensen, from Yale University.

which one is more easy to use when it comes to infer the force field from QM results?

I cannot answer this question directly because I never did such calculations, but the functional form looks straight forward to be parametrized from QM calculations. A rapid information can be obtained from the Wiki site and details about it from the reporting paper:

William L. Jorgensen, David S. Maxwell, and Julian Tirado-Rives. Development and Testing of the OPLS All-Atom Force Field on Conformational Energetics and Properties of Organic Liquids J. Am. Chem. Soc. 118 (45): 11225–11236 (1996). DOI: 10.1021/ja9621760. CiteSeerX 10.1.1.334.2959

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