[gmx-users] How to increase the number of parameters in dihedral angle potentials?

[bhong at Princeton.EDU]

Dear all,

This is the problem I meet with. GROMACS provides a Ryckaert-Bellemans function with 6 parameters (C0, C1, C2, ..., C5) , while what I need is a function of summation over 8 terms, that is, 8 parameters from C0 to C7. I tried to add the values of  the two additional pamameters at the end of each lines in the section of dihedral angles in the 'molecule.itp' file. The 'grompp' command then gave an error information, saying there could only be either 6 or 12 parameters for Ryckaert-Bellemans potentials. So I added 4 zeros for the rest 4 parameters in 'molecule.itp'. Now, the 'grompp' command did not gave error information any more and I run the simulation with a 12-parameter Ryckaert-Bellemans potential. The problem is that the trajectory obtained from 12-parameter RB potential is exactly the same as the trajectory having generated by 6-parameter RB potential. That means the newly added 6 parameter values can not be recognized by GROMACS. Has anyone met with such kind of problems? What did you do to make GROMACS recognize 6 more parameters in RB potentials? 

Thanks
Bingster