[gmx-users] Re: abnormal flip-flop when embedding cholesterol into DOPC using CG

H.J.Risselada H.J.Risselada at rug.nl
Thu Dec 6 11:32:42 CET 2007

Dear Ran,

Indeed, strictly speaking a smaller time step will give you a more 
precise intergration over phase-space. However, as the CG-model itself 
and theirfore its corresponding energy description are parametrized 
for only a certain range in time steps. The extra details which one 
with a more precise intergration over the energy field by chosing a 
much smaller time step are beyond the actual "resolution" of the 
CG-model and therefore not physical.

.........About the flip-flop problem. Please be more precize. Describe 
what is a high rate in flip-flops and how long you have simulated. The 
change of DOPC in cholesterol will give a dramatic change in the area 
per lipid. What one sees is probably a system trying to equlibrate 
itself. Does the problem still continue, after lets say 50 ns ?

Good luck,



On Thu, 06 Dec 2007 10:17:42 +0100
  Ran Friedman <r.friedman at bioc.uzh.ch> wrote:
> Dear Xavier, GMX users,
> Xavier Periole wrote:
>> One other thing. In the CG FF the time step is considered as part of 
>> force field. Meaning that you should not play with it too much.
>> 0.04-0.02 ps is the actual range where the force field is considered
>> to reproduce experimental values of various quantities used for
>> paraameterization of the FF. Outside this range you change the 
>> between the different terms and thus would alter the quality of the 
> Can you elaborate on this a bit? Surely if one changes the 
> pressure, compressibility, threshold distances etc., one doesn't
> necessarily reproduces the experimental results. Too long timesteps 
> also cause a problem. But why would it be wrong to use a shorter 
> step here?
> Ran.
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