[gmx-users] Energy minimization has stopped

[Justin Lemkul]

On 6/23/16 6:35 AM, Anurag Dobhal wrote:
> 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.
>

What does this mean?  Are you using OPLS-AA or CHARMM27?

> 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 ?
>

http://www.gromacs.org/Documentation/Errors#Stepsize_too_small.2c_or_no_change_in_energy._Converged_to_machine_precision.2c_but_not_to_the_requested_precision

mdrun prints which atom is feeling the maximum force, so that's a good place to 
start looking to see if there is an actual problem.

-Justin

-- 
==================================================

Justin A. Lemkul, Ph.D.
Ruth L. Kirschstein NRSA Postdoctoral Fellow

Department of Pharmaceutical Sciences
School of Pharmacy
Health Sciences Facility II, Room 629
University of Maryland, Baltimore
20 Penn St.
Baltimore, MD 21201

jalemkul at outerbanks.umaryland.edu | (410) 706-7441
http://mackerell.umaryland.edu/~jalemkul

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