[gmx-users] Steepest Descents did not converge

[Christoph Freudenberger]

Derrick Guang Yuh Lee wrote:
> dear gmx-users
> 
> right now i am running the mdrun program in gromacs on a dppc lipid
> bilayer (downloaded from the tieleman's website), and i keep on getting
> this error:
> 
> ---------------------------------------------------------------------------
> Steepest Descents:
>    Tolerance         =  1.00000e+02
>    Number of steps   =      1000000
> Step=  0, Dmax= 1.0e-02 nm, Epot= -2.99255e+05 Fmax= 5.37264e+03, atom=3275
> Step=  1, Dmax= 1.0e-02 nm, Epot= -3.02471e+05 Fmax= 2.40785e+03, atom=3024
> Step=  4, Dmax= 3.0e-03 nm, Epot= -3.02866e+05 Fmax= 1.90722e+03, atom=13315
> Step= 16, Dmax= 1.8e-06 nm, Epot= -3.02866e+05 Fmax= 2.63245e+03, atom=16161
> 
> Stepsize too small (8.78906e-07 nm)Converged to machine precision,
> but not to the requested precision (100)
> You might need to increase your constraint accuracy, or turn
> off constraints alltogether (set constraints = none in mdp file)
> 
> writing lowest energy coordinates.
> 
> Steepest Descents did not converge in 17 steps
>   Potential Energy  = -3.02866e+05
> Maximum force:  1.90722e+03
> ---------------------------------------------------------------------------

This is not an error, steepest decent has just reached it's limit.
If you want to start an MD for the system just
proceed to a short run in NVT with a rather short time step (1fs or
maybe even less). If the system runs fine you can turn on pressure
coupling (if you want to) and increase the time step.

If you want to do further minimization switch over to cg.

regards
-- 
Christoph Freudenberger
Abt. Organische Chemie I AK Siehl - Uni Ulm -Tel: ++49-731-502-2785