[gmx-users] infinitive force acting on atom during minimization

[Joern Lenz]

dear users,
i try to simulate a system of DNA (17 bp) and a protein (420 aa).

before i start my MD runs, i solvated the system in a cubic box (-d = 0.9) and 
started a minimization of it using the em.mdp file which you can see at the 
bottom of this email.
unfortunately the minimization breaks (see below).
And if it does not break, it is very slow (20 iterations take about 3 weeks). 
Can you see from the information i give to you, why this happens?
The output of this minimizations looks something like:

Steepest Descents:
Tolerance (Fmax)   =  1.00000e+00
Number of steps    =          400
Step=    0, Dmax= 5.0e-04 nm, Epot=  4.14463e+16 Fmax=  inf, atom= 2005
Step=    1, Dmax= 5.0e-04 nm, Epot= nan Fmax= 0.00000e+00, atom= 0
Step=    2, Dmax= 2.5e-04 nm, Epot= nan Fmax= 0.00000e+00, atom= 0
Step=    3, Dmax= 1.3e-04 nm, Epot= nan Fmax= 0.00000e+00, atom= 0
Step=    4, Dmax= 6.3e-05 nm, Epot= nan Fmax= 0.00000e+00, atom= 0
...

Hence there is an infinitive force acting on atom 2005 in my system, right ?
First I looked at the coresponding file to figure ot which residue this atom 
belongs to. I then looked at the system using chimera as a viewer and found 
that is residue occurs several times in my system, which worries me a little 
bit. But there is no such thing like two atoms lying very close to each other 
which would explain the Fmax=  inf, atom= 2005 line in the minimization.
Has anyone got an idea of what is going wrong here or do you need more 
information about my used files.

It would be of great help for my further work, so thanks a lot in advance and 
have a nice weekend
Greetings

====================
em.mdp:

title	        	= Minimization of blah.pdb
cpp  		= /usr/bin/cpp
define     	= -DFLEX_SPC
integrator     	= steep
nsteps		= 400 
nstenergy	= 5      
nstxtcout	= 5
nstlist		= 5
ns_type         	= simple
rlist           	= 1.0
coulombtype	= PME
rcoulomb	= 1.0
rvdw		= 1.0
pbc			= xyz
fourierspacing	= 0.12
fourier_nx	= 0
fourier_ny	= 0
fourier_nz	= 0
pme_order	= 4
ewald_rtol	= 1e-5
optimize_fft	= yes
constraints	= none
emtol		= 1.0
emstep		= 0.005
====================