[gmx-users] Energy minimization has stopped

[Anurag Dobhal]

Dear Gromacs Users, I am Simulating a  RNA molecule using OPLS AA force
field. I have written parameters using CHARMM27 force field for RNA
molecules.

Minimising the molecule by invoking mdrun gives me the follwoing error.

Energy minimization has stopped, but the forces have not converged to the
requested precision Fmax < 1000 (which may not be possible for your system).
It stopped because the algorithm tried to make a new step whose size was too
small, or there was no change in the energy since last step. Either way, we
regard the minimization as converged to within the available machine
precision, given your starting configuration and EM parameters.

Double precision normally gives you higher accuracy, but this is often not
needed for preparing to run molecular dynamics.
You might need to increase your constraint accuracy, or turn
off constraints altogether (set constraints = none in mdp file)

-----------------------------------------------------------------------------------------------------
The contents of the .mdp file are the following

; minim.mdp - used as input into grompp to generate em.tpr
integrator = steep ; Algorithm (steep = steepest descent minimization)
emtol = 1000.0   ; Stop minimization when the maximum force < 1000.0
kJ/mol/nm
emstep      = 0.01      ; Energy step size
nsteps = 50000   ; Maximum number of (minimization) steps to perform

; Parameters describing how to find the neighbors of each atom and how to
calculate the interactions
nstlist    = 1    ; Frequency to update the neighbor list and long range
forces
cutoff-scheme   = Verlet
ns_type    = grid ; Method to determine neighbor list (simple, grid)
coulombtype    = PME ; Treatment of long range electrostatic interactions
rcoulomb    = 1.0 ; Short-range electrostatic cut-off
rvdw    = 1.0 ; Short-range Van der Waals cut-off
pbc        = xyz ; Periodic Boundary Conditions (yes/no)

any suggestions ?


*Anurag Dobhal*
*Graduate Student (Bioprocess Technology)*
*Institute of Chemical Technology, Mumbai*

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