CCL:G: MD with DREIDING FF: system becomes unstable on inclusion of charges

 Sent to CCL by: pascal boulet [pascal.boulet]-[univ-provence.fr]
 Hello,
 When you say: "I checked, the values are reasonable and similar to
 another published paper " do you mean that the charges are reasonable or
 that the combination of the charges with the Dreiding FF are reasonable?
 The reason I ask the question is that probably you are "messing up"
 your
 FF by using these charges. They are probably not compatible with each
 other. Usually, FF parameters are fitted in conjunction with some
 specific charges in order to get an overall satisfactory force field. If
 you then change the charges the FF is no longer self-consistent.
 I think that Amber has been designed to work with ab initio charges.
 Perhaps you can adapt Amber parameters to your needs using your charges.
 Another option is to get the adequate charges for the Dreiding FF, if
 there are.
 You can get rid of the charges only if you have non polar chains in your
 polymer. otherwise, you will not describe properly e.g., hydrogen bonds
 (either intra or inter-molecular).
 If I were you, I would change the Dreiding FF for, e.g., cff91 or cff94,
 if compatible with DL_Poly. But, you first have to check if the
 corresponding energy terms are implemented in the program.
 [A]ARTICLE{Rappe1991,
   author = {Anthony K. Rappe and William A. Goddard},
   title = {Charge equilibration for molecular dynamics simulations},
   journal = {The Journal of Physical Chemistry},
   year = {1991},
   volume = {95},
   pages = {3358--3363},
   number = {8},
   month = apr,
   abstract = {approach for predicting charge distributions in molecules
 for use
     in molecular dynamics simulations. Experimental atomic ionization
     potential, elctron affinities, atomic radii. Atomic chemical potential.
     shielded electrostatic interactions. Charge equilibration {QEq.}
     {CFF91} force field.},
   doi = {10.1021/j100161a070},
   keywords = {{CFF91}},
   url = {http://dx.doi.org/10.1021/j100161a070}
 }
 Hope this help,
 Pascal
 Le 16/08/2011 10:20, Emmeline Yeo emma.yeo2704_._gmail.com a écrit :
 > Sent to CCL by: "Emmeline  Yeo" [emma.yeo2704|-|gmail.com]
 > Hi everyone,
 >
 > I'm new to force field molecular dynamics and will appreciate any advice
 that
 >
 > you can give on problem that I have.
 >
 >
 > I was trying to simulate an amorphous polyacetylene polymer cell using the
 >
 > DREIDING force field with the program dl_poly. The system is stable and
 runs
 >
 > well if I set the atomic charges for all my atoms to zero. However, once I
 >
 > include the atomic charges (calculated using Gaussian program - and I
 >
 > checked, the values are reasonable and similar to another published paper),
 >
 > the system just becomes out of control: the total energy and the
 temperature
 >
 > of the system becomes very high.
 >
 >
 > I don't think the problem lies with a bad initial configuration for the
 >
 > polymer as the same thing happens even if I just have 2 polymer chains in
 my
 >
 > periodic cell.
 >
 >
 > An advice that I've been given is to ignore atomic charges except in the
 >
 > cases of simulating polyelectrolytes...
 >
 >
 > Any other suggestions given is deeply appreciated!>
 >
 >
 --
 !!!!WARNING: my phone number has CHANGED!!!!
 Dr. pascal Boulet, computational chemist
 University of Aix-Marseille I
 Laboratoire Chimie Provence, UMR 6264
 Group of Theoretical Chemistry
 Avenue Normandie-Niemen
 13397 Marseille Cedex 20
 France
 **********
 Tel. (+33) (0)413.55.18.10
 Fax. (+33) (0)491.55.18.50
 **********
 http://www.lc-provence.fr
 https://sites.google.com/a/univ-provence.fr/pb-comput-chem
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