[gmx-users] EM problem with OPLS and TIP4P

[chris.neale at utoronto.ca]

Try this:
unconstrained_start = no

Also ensure that your .top file is correct.

Also take a look at your starting positions, where is MW?

Also ensure that your waters have the correct order:
eg.
   871SOL     OW 3496   1.935   2.093   1.901
   871SOL    HW1 3497   1.902   2.172   1.945
   871SOL    HW2 3498   1.990   2.126   1.830
   871SOL     MW 3499   1.938   2.107   1.898

Also try outputting coords for step 20,21,22,23 in the first system  
and see what is actually happening.