[gmx-users] Stepsize too small ( what to do?)

li dawei dli at clarku.edu
Wed Aug 3 14:26:42 CEST 2005

I guess there are sth wrong in your initial simu cell.

Energy  minimzation cannot guarantee a "good" initial structure at this 

----- Original Message ----- 
From: "Anil Kumar" <anil at chem.iitb.ac.in>
To: <gmx-users at gromacs.org>
Sent: Tuesday, August 02, 2005 12:46 PM
Subject: [gmx-users] Stepsize too small ( what to do?)

Hi to all,

I am doing normal MD simulation:- vac followed by em: followed  by pr: and
then finally full MD.

But i am getting a problem like this during energy minimization by
steepest descents,

"Stepsize too small, or no change in energy.
Converged to machine precision,
but not to the requested precision Fmax < 100

Double precision normally gives you higher accuracy.
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 converged to machine precision in 34 steps,
but did not reach the requested Fmax < 100.
Potential Energy  = -8.0718016e+04
Maximum force     =  1.9289487e+03 on atom 4826

I have changed the em step in em.mdp file but still it is not working.
Secondly i have started pr with the em conformation, but it crashes.

any suggestion !!
thanks in advance
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