[gmx-users] EM problem

[sgaray at fbcb.unl.edu.ar]

Dear Gromacs user´s

My name is Sergio Garay  and I´m trying to minimize de E of a 
bilayer of 72 stearic acid in a periodic boundary conditions. I´m 
using ffG43a2x forcefield with and without position restraints in C of 
carboxilic acids.
Grompp preprocesator doesn´t give any warning but the EM doesn´t 
work properly (log below). It doesn´t seems to work :

Stepsize too small (7.62939e-07 nm)Converged to machine 
precision,
but not to the requested precision (200)

Steepest Descents did not converge in 18 steps
  Potential Energy  =  9.69202e+17
Maximum force:          inf

	M E G A - F L O P S   A C C O U N T I N G

   RF=Reaction-field  Free=Free Energy  SC=Softcore
   T=Tabulated        S=Solvent         W=Water     WW=Water-Water

           Computing:      M-Number      M-Flop's  % Flop's
                   LJ      3.521646    109.171026    15.4
         LJ + Coulomb      0.006480      0.246240     0.0
           Coulomb(T)      0.044208      1.812528     0.3
      LJ + Coulomb(T)      0.132624      7.161696     1.0
      Innerloop-Iatom      0.142542      1.425420     0.2
     Spread Q Bspline      1.741824      3.483648     0.5
     Gather F Bspline      1.741824     20.901888     3.0
               3D-FFT     47.746584    381.972672    54.0
            Solve PME      0.731808     46.835712     6.6
             NS-Pairs      5.764806    121.060926    17.1
         Reset In Box      0.027216      0.244944     0.0
              Shift-X      0.027216      0.163296     0.0
               CG-CoM      0.023328      0.676512     0.1
                Bonds      0.025920      1.114560     0.2
               Angles      0.025920      4.224960     0.6
              Propers      0.005184      1.187136     0.2
            Impropers      0.001296      0.269568     0.0
         RB-Dihedrals      0.019440      4.801680     0.7
               Virial      0.000486      0.008748     0.0
          Total                   706.76316   100.0

               NODE (s)   Real (s)      (%)
       Time:     30.000     30.000    100.0


When I tried to run a MD simulation starting with the EM_out.gro file 
the system crashes (I guess, the system has very bad contact 
between many atoms).

Also, I tryied a system with a stearic acid alone in vacuo and the EM 
work fine (make some steps and change the potential Energy of the 
system).

Could you give any advice?

Thank You.

Sergio