I want to use gromacs calculate Li ion interaction with ion channel.I want to use CHARMM force field.but no Li parameter in CHARMM force field. I find Li parameter in OPLS/AA force field. I want to known how to transform sigma and epsilon parameter between these two force field.
Thank you
You can't simply transform the parameters from one force field to another. Force fields are self-consistent entities and need to be regarded as such.
CHARMM36 supports Li+ (atom type LIT); obtain the parameters from http://mackerell.umaryland.edu/charmm_ff.shtml#gromacs
Li+ is just not commonly used. Add an entry for it in ions.itp yourself just like any other entry.
AMBER has a new set of force field for ions (mono-, di-, tri-). The force field has been testified by experimental hydration free energy, ion water distance and coordination number. You may check more information in AMBERTOOLS15 manual. AMBER14 is free to use on CPU. You may use AMBER rather than Gromacs for this project.
Ruan, there is definitely a Li+ in the CHARMM force field. Here is a link to the CHARMM 36 force field, which has been prepared for gromacs.
http://mackerell.umaryland.edu/charmm_ff.shtml#gromacs
Dear Justin
I have a question about the charge value about charmm36 force field. I have download package from http://mackerell.umaryland.edu/charmm_ff.shtml#gromacs. I check the file ffnonbonded.itp, found that charge for atoms in [atomtypes] is 0.000. I want to known how t deal with the problem.
Thank you.
Charges in ffnonbonded.itp are not used by anything. It's a relic of an old file format.
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