

Dear Tahmid,

Do you know what is the maximum allowable time step
for the integrator you are using ?

Note that most probably the method you are using is
only "conditionally stable" i.e. stable for small
time step and unstable for time step grater then
certain treshold.

For more detail analysis you may refer any
handbook devoted to MD and/or numerical
solutions for differential equations
e.g. J.M. Haile "Molecular Dynamics Simulation: Elementary Methods"
J. Wiley & Sons, Inc.
and most probably Allen & Tildesley as well.

Of course this does not exclude possibility
of the bug in your implementation - unfortunately.

Sincerely,

Ryszard Czerminski, MSI
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-------   Regard
          Tahmid Mizan