Dear Justin, Thank you for your reply. I went trough the CHARMM forum and I found this post of 2007, https://www.charmm.org/ubbthreads/ubbthreads.php?ubb=showflat&Number=28545, which basically underlines the same issues I am having. I read this post before posting here, and this why I came with the idea of modifying the dihedral ff parameters, as suggested in the forum. Apparently, it may work if I set the amplitude k=0 and I distort the linear angle a little (maybe 179.9) in the initial structure. The thing is, as I say before, I have no experience modifying force fields, so I wouldn't what to put and where. My guess is that I would need to go the ffbonded.itp file and modify and add the following lines in this way:
[ angletypes ] ; i j k func theta0 ktheta ub0 kub CG331 CG1N1 NG1T1 5 179.900000 177.401600 0.00000000 0.00 [ dihedraltypes ] ; i j k l func phi0 kphi mult HGA3 CG331 CG1N1 NG1T1 9 0.000000 0.000000 1 [ dihedraltypes ] ; 'improper' dihedrals ; i j k l func phi0 kphi HGA3 CG331 CG1N1 NG1T1 9 0.000000 0.000000 1 Please let me know if this makes any sense. Is it correct to modify the angletype or should I only modify the initial structure? Is there someone who has already done simulations with acetonitrile? Any advice and feedback is very much welcome!! Thank you very much, Have a nice day, Sonia Aguilera PhD Student ENSCM > > Message: 2 > Date: Wed, 5 Jul 2017 17:47:08 -0400 > From: Justin Lemkul <jalem...@vt.edu> > To: gmx-us...@gromacs.org > Subject: Re: [gmx-users] gromacs.org_gmx-users Digest, Acetonitrile > with CHARMM ff > Message-ID: <c935a574-ffc8-cf5f-3a3c-3131d906a...@vt.edu> > Content-Type: text/plain; charset=utf-8; format=flowed > > > > On 7/5/17 10:47 AM, Sonia Milena Aguilera Segura wrote: > >> On 7/5/17 5:15 AM, Sonia Milena Aguilera Segura wrote: > >>> Dear GROMACS users, > >>> > >>> I am currently interested in studying properties of some solvents, among > >>> them acetonitrile and isopropanol. I would like to use CHARMM force field > >>> for compatibility with other molecules and I am taking my initial > >>> structure files from virtualchemistry.org. Does someone know how to run > >>> the 6-sites model available with the CHARMM ff in gromacs? As I try to > >>> run > >>> the simulation box I get the error "No Defaul Proper Dih. Types". I > >>> checked the ffbonded file and I didn't see the dihedrals defined as in > >>> the > >>> rtp file. And for what I've understood this is > >> > >> Dihedrals aren't required in .rtp files since pdb2gmx generates them. > > > > > > Dear Justin, > > > > Thank you for the comments. Yes, sorry, I was referring to the top file > > generated by pdb2gmx instead of the rtp file. The top file has the > > dihedrals defined whereas ffbonded files does not. > > > > > >> > >> because acetonitrile is a linear molecule and dihedrals for three colinear > >> atoms > >> this are mathematically > >> > >> There are still H-C-C-N dihedral terms. > > > > > > Yes, they are named in the top file but not really defined in the ffbonded > > file, which is why I get the error. > > > > pdb2gmx will blindly generate all possible dihedrals; sometimes this isn't > right. > > It even appears that such parameters (H-C-C-N) do not exist in any CHARMM > force > field file. Perhaps it's meant that way, but I have never tried to simulate > anything with it. > > > > >> > >> undefined. Also, when I go to check the available itp for acetonitrile in > >> virtualchemistry.org, I can see that there is a 7-sites model,with a dummy > >> atom > >> included. However, for this case, pdb and itp files do not match. I have > >> seen > >> that this can be sort of solved by fixing the angle as 179.9, but I really > >> don't > >> exactly what to change > >>> or where in the force field files. I have no experience modifying > >>> force > >>> fields. Also, I've seen that for > >> > >> Don't modify the force field. The CHARMM parameters for acetonitrile were > >> generated in CHARMM, which can handle linear angles without the tricks > >> that > >> GROMACS requires with virtual sites. > > > > > > So, what you are saying is that it is not possible to simulate the 6-sites > > model of acetonitrile in GROMACS using CHARMM? > > Never tried it. AFAIK, GROMACS needs special treatment for linear species. > Hopefully someone who has actually done such a simulation will say something, > because my usefulness here is at its end :) > > -Justin > > > > >> > >> the opls ff both pdb and itp files match, but I really need to use the > >> CHARMM > >> force field. Are the opls parameters compatible with CHARMM? Any advice? > >>> > >> > >> Don't mix and match force fields. > > > > > > Then, if I cannot run the 6-sites model, what's the advice? I would really > > need to run acetonitrile with CHARMM, because I have other series of > > molecules such as cellulose and other saccharides. In that case, what > > would be the most compatible for field to simulate organic solvents and > > saccharides? > > > >> > >>> Also, I cannot find an already optimized structure with all hydrogens for > >>> isopropanol. Could somenone provide one? > >> > >> Take any valine side chain and use it. You can generate any missing H > >> easily > >> with a suitable .hdb entry. > >> > >> -Justin > > > > Thank you for your advice, > > > > Sonia Aguilera > > PhD student > > ENSCM > > -- Gromacs Users mailing list * Please search the archive at http://www.gromacs.org/Support/Mailing_Lists/GMX-Users_List before posting! * Can't post? Read http://www.gromacs.org/Support/Mailing_Lists * For (un)subscribe requests visit https://maillist.sys.kth.se/mailman/listinfo/gromacs.org_gmx-users or send a mail to gmx-users-requ...@gromacs.org.