[gmx-users] Stepsize too small, or no change in energy

Gale, Ella ella.gale at imperial.ac.uk
Tue Jul 18 10:57:43 CEST 2006

Dear Dhiraj,

You're using single precision GROMACS which means that there is a limit to how small your forces can go, if you want to minimise further you will need to use double precision GROMACS (which will decrease the machiene precision allowing you minimisation to run for longer). You will have to recompile GROMACS to double precision and reinstall it following the instructions in the manual. Then you call the higher precision GROMACS by appending an '_d' to the end of the names of all the programs (eg grompp_d and mdrun_d).


Ella Gale

-----Original Message-----
From: gmx-users-bounces at gromacs.org on behalf of Srivastava, Dhiraj (UMC-Student)
Sent: Tue 7/18/2006 6:13 AM
To: Discussion list for GROMACS users
Subject: [gmx-users] Stepsize too small, or no change in energy
Hi all

      when i am trying to minimize my structure, i am getting following error.

Stepsize too small, or no change in energy.
Converged to machine precision,
but not to the requested precision Fmax < 1000

Double precision normally gives you higher accuracy.

writing lowest energy coordinates.

Back Off! I just backed up prodh_em.trr to ./#prodh_em.trr.2#

Back Off! I just backed up prodh_wb.pdb to ./#prodh_wb.pdb.4#

Steepest Descents converged to machine precision in 45 steps,
but did not reach the requested Fmax < 1000.
Potential Energy  = -1.8373166e+06
Maximum force     =  1.5158329e+04 on atom 519
Norm of force     =  3.1759754e+04

i have tried the conjugated gradient, constraint none and also included flexible water but still i am getting the same error.

what should i do to solve this problem?

Thanks in advance for help.

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