Well.
I think I agree with David.... 100 cations and 100 anions without
solvent (am I correct?)... probably your system is freezing and an
evidence of this is that the slope of your MSD graph tends to zero, so
your D tends to zero too.
But considering what you said about ACFs and RDFs, I think you know what
you are doing and I´ll talk about only MSD.
Your MSD plot is right for me. It starts with a non-linear part and
after (near 1.5 ns) it become reasonable linear. But I think you need
more statistic and I suggest you run more 5 ns of simulation the get a
better linear part.
The differences between values of D calculated with and without fitting
is that in the first case you are fitting the Einstein equation (see
manual) in hole graph. In the second case you are using times 0-20 and
fitting the equation only in this part of the graph and, like you can
see on it, the curve have a high slope in the beginning. Actually, you
should use the begin-endfit option from 1.5 ns until the end of graph to
fit the Einstein equation only to the "linear" part.
About -trestart, it works restarting the initial position of the
molecule every X ps. Try to use 1, 5 and 10 ps values for it and see
what happen.
Well... summarizing... try to run more 5 ns of simulation, run msd using
-beginfit from the begin of linear part of your graph, play with
t_restart values.
I hope I helped you.
Cheers
Alexandre Suman de Araujo
[EMAIL PROTECTED]
UIN: 6194055
IFSC - USP - São Carlos - Brasil
Roman Holomb wrote:
Dear colleagues,
Thanks for reply!
Maybe I'm doing something wrong with using g_msd .....
I have computed MD (5ns) for system containing 100 anion and 100
cation molecules.
Results of ACFs, RDFs look good but not MSD....
So I have 2_emi_BF4_100.tpr, 2_emi_BF4_100.trr, 2_indexEMI.ndx files.
Index file contained numbers of EMI-cation molecules.
Then I use command:
g_msd -f 2_emi_BF4_100.trr -n 2_indexEMI.ndx -s 2_emi_BF4_100.tpr -o
2_emi_BF4_100_msd_EMI_full.xvg -mol 2_emi_BF4_100_msd_full_D_EMI.xvg
And I have resulted Diffusion constants fitted from 0 to 5000 (ps)
D[EMI]=(0.002+/-0.003)*10-5 cm2/s
The resulted MSD graph is included.
Then I try to use time 0-20 ps for fitting:
g_msd -f 2_emi_BF4_100.trr -n 2_indexEMI.ndx -s 2_emi_BF4_100.tpr
-beginfit 0 -endfit 20 -o 2_emi_BF4_100_msd_EMI_0_20.xvg -mol
2_emi_BF4_100_msd_0_20_D_EMI.xvg
The resulted D: 0 to 20 (ps) D[EMI]=(0.508+/-0.253)*10-5 cm2/s. But
MSD graphs look ideally the same.....
I'm not sure that program read number of molecules (instead of atoms).
option -mol give only D for different molecules but not their average
MSD....
Also -trestart option dosn't work and give me "Segmentation Fault".......
Any comments are advisable.
Thanks in advance!
Sincerely
/Roman
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