# [gmx-users] Energy minimisation with different water models

David spoel at xray.bmc.uu.se
Fri Apr 22 22:03:01 CEST 2005

```On Fri, 2005-04-22 at 19:51 +0000, Tomek R wrote:
> solution to it. I am running a simple steepest descent energy minimisation
> (em.mdp input file like in speptide tutorial) on a AAA tripeptide /OPLSAA
> forcefield, GROMACS 3.2.1 and I am getting the following results on
> different water models:
>
> It converges well with SPC / TIP5P, but something goes very wrong with
> TIP4P. What's the problem?
>
probably a Hydrogen was fatally attracted to a negative charge (or
oxygen). If the H is close enough to an O there is a singularity in the
potential... Try slightly different starting coordinates for the atom

> Tomek
>
> SPC:
> Step=   42, Dmax= 1.6e-02 nm, Epot= -3.86495e+04 Fmax= 1.11394e+04, atom= 5
> Step=   44, Dmax= 9.6e-03 nm, Epot= -3.88441e+04 Fmax= 1.82268e+03, atom= 5
> .
> .
> Steepest Descents converged to Fmax < 2000 in 45 steps
> Potential Energy  = -3.8844137e+04
> Maximum force     =  1.8226837e+03 on atom 5
> Norm of force     =  7.5002910e+03
>
> TIP4P:
> Step=   28, Dmax= 2.7e-06 nm, Epot= -1.39090e+08 Fmax= 2.16801e+14, atom=
> 1656
> Step=   30, Dmax= 1.6e-06 nm, Epot= -3.75909e+07 Fmax= 2.45911e+13, atom=
> 1656
> Stepsize too small, or no change in energy.
> Converged to machine precision,
> but not to the requested precision Fmax < 2000
>
> Double precision normally gives you higher accuracy.
> .
> .
> Steepest Descents converged to machine precision in 31 steps,
> but did not reach the requested Fmax < 2000.
> Potential Energy  = -1.3909030e+08
> Maximum force     =  2.1680114e+14 on atom 1656
> Norm of force     =  2.8675946e+14
>
>
> TIP5P:
> Step=   33, Dmax= 7.5e-03 nm, Epot= -9.64603e+03 Fmax= 4.13647e+03, atom= 35
> Step=   35, Dmax= 4.5e-03 nm, Epot= -9.67248e+03 Fmax= 1.89664e+03, atom= 35
> .
> .
> Steepest Descents converged to Fmax < 2000 in 36 steps
> Potential Energy  = -9.6724756e+03
> Maximum force     =  1.8966361e+03 on atom 35
> Norm of force     =  2.5414328e+04
>
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--
David.
________________________________________________________________________
David van der Spoel, PhD, Assoc. Prof., Molecular Biophysics group,
Dept. of Cell and Molecular Biology, Uppsala University.
Husargatan 3, Box 596,          75124 Uppsala, Sweden
phone:  46 18 471 4205          fax: 46 18 511 755
spoel at xray.bmc.uu.se    spoel at gromacs.org   http://xray.bmc.uu.se/~spoel
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