[gmx-users] hints for core/shell optimization?

Tamas Karpati tkarpati at gmail.com
Fri Jul 25 13:43:25 CEST 2014

Dear all,

I have two questions about geometry optimization of a crystal
with polarization via the core/shell model. I'm creating *.gro and
*.top files "by hand" and compile them with *.mdp to *.tpr via
GROMPP. My FF is also made by hand (simply because i need
to learn GROMACS). I have learnt on this list that with Buckingham
potentials I need to use the group rather than the Verlet scheme.

(1/2) Letting some of all atoms be polarizable through applying
shell particles made MDRUN segfault like this:

#  ...
#  Reading file AAA_opt.tpr, VERSION 4.6.3 (single precision)
#  Using 2 MPI threads
#  Steepest Descents:
#     Tolerance (Fmax)   =  1.00000e+01
#    Number of steps    =           10
#  Segmentation fault1.0e-02 nm, Epot=         -nan Fmax= 3.76506e+03,
atom= 1357

I imagined some divison by zero situation not handled and have put
some "random noise" on the shell particles' position so they do not
anymore start exactly at the atomic sites (meaning nonzero distances).
Seemed to work, at least no further crashes. Only energies and forces
seem very high:

#  Steepest Descents:
#     Tolerance (Fmax)   =  1.00000e+01
#     Number of steps    =           10
#  Step=    0, Dmax= 1.0e-02 nm, Epot= -1.36425e+07 Fmax= 2.99600e+05, atom= 160
#  Step=    1, Dmax= 1.0e-02 nm, Epot= -1.62080e+07 Fmax= 1.25769e+06, atom= 160
#  Step=    2, Dmax= 1.2e-02 nm, Epot= -1.95965e+07 Fmax= 6.87820e+08,
atom= 2759
#  Step=    3, Dmax= 1.4e-02 nm, Epot= -2.02902e+07 Fmax= 1.30719e+09, atom= 468
#  Step=    8, Dmax= 1.1e-03 nm, Epot= -2.18970e+07 Fmax= 5.77722e+09,
atom= 1095
#  Step=   10, Dmax= 6.5e-04 nm, Epot= -1.96952e+07 Fmax= 3.92889e+08,
atom= 1096
#  Energy minimization reached the maximum numberof steps before the forces
#  reached the requestedprecision Fmax < 10.

My question is the following.
   Is it (randomized shell positions) a correct procedure with GROMACS?

(2/2) Changing from a randomized x/y/z set to a fixed distance at
a random direction for the shell particles led to another unexpected result.
I scanned a range between 1e-4 to 0.1 nm and noticed that

    the final core-to-shell distance is a function of the starting one.

I used niter = 10000 (note: the default is 20) as i noticed in an MD
type of job that 20, 100 or 1000 steps were insufficient for the shells
to relax within default tolerance. The cell size was ca. 3x3x3 nm.

My question is the following.
   What would be the appropriate core-to-shell distance to apply?

I appreciate any help so thanks in advance.

With regards,

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