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I am trying to separate two methyl-N,N'-diacetylchitobiose sugars using steered molecular dynamics in AMBER. I have tested out:

&rst
iat=-1,-1,
r2=2.5,
rk2=10.0,
r2a=12.5,
iresid=0,
fxyz=1,1,1,
outxyz=1,
igr1=2,5,6,8,9,11,14,16,18,20,22,24,26,27,28,32,33,35,36,38,41,43,45,47,49,51,53,54,55,59,60,
igr2=65,68,69,71,72,74,77,79,81,83,85,87,89,90,91,95,96,98,99,101,104,106,108,110,112,114,116,117,118,122,123
/

Which should apparently separate the two sugars relative to their heavy atoms from a distance (between each other) of 2.5 A to 12.5 A along the x,y and z axes and using a force constant of 10 (I think kcal/mol). The solvent I am using is water.

I ran this separation for 100 ns and the two sugars refused to separate. I am not sure if my force constant is too small, or if something else is wrong with my reference file. I have googled it and spoken to m supervisor about it, and unfortunately I have not succeeded in finding a solution.

Any help would be appreciated.

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    $\begingroup$ Are you sure you have specified the correct groups/residue IDs/atom numbers? If yes, then have you tried increasing the force constant to a very high value, to see whether or not the molecules separate? $\endgroup$ – Bdrs Jun 6 '18 at 8:53
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    $\begingroup$ The force constant was too low. I managed to fix it now. $\endgroup$ – LLCJ27 Jun 6 '18 at 10:32
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I have realized the problem. The force constant was too low. Increasing it to 25.0 from 10.0 sorted things out.

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