[gmx-users] Very large Max Force [Energy minimisation] in water with ions

Erik Marklund erik.marklund at chem.ox.ac.uk
Thu Nov 6 12:36:05 CET 2014

Dear Kester,

The potential energy is highly positive in the first case and the force is enormous in the second case, so no wonder that they blow up. How did you prepare these systems?

Kind regards,

Erik Marklund, PhD
Postdoctoral Research Fellow, Fulford JRF

Department of Chemistry
Physical & Theoretical Chemistry Laboratory
University of Oxford
South Parks Road

On 6 Nov 2014, at 04:31, Kester Wong <Kester2014 at ibs.re.kr<mailto:Kester2014 at ibs.re.kr>> wrote:

Dear all,

I have been trying to energy minimise ~1980 water (tips3p) molecules with 20 OH- anions (with virtual sites and dummy atoms) and 20 Na+ cations, however, I always ended up getting a very large Max Force.

Using emtol = 1000 and emstep = 0.0001, I get:

Potential Energy  =  3.79607808453231e+09
Maximum force     =  9.86741557582003e+02 on atom 42394
Norm of force     =  2.38846729392462e+01

Using emtol = 700 and emstep = 0.0001, I get:

Potential Energy  = -6.43566327273198e+13
Maximum force     =  9.24568347101016e+25 on atom 42262
Norm of force     =  5.74744822148669e+23

As expected, both structures blew up during NVT equilibration.

Could this be due to the arrangement of the ions? I tried separating the anions and cations in two boxes (placed on top of the water droplet), but that also yield a very large Max Force.

FYI, the water droplet on graphene was equilibrated for 20ns.
Any input is greatly appreciated. Thanks.

- Kester

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