

Hello Tahmid,

In the case of the Verlet algorithm and nonbonded atoms the blowing up
is a typical behaviour when the time step is increased. See for
example the User Guide, Part 1 for Discover 2.9/3.1, January 1993,
Biosym, pp.  1/2-58 - 1/2-61: As the two nonbonded atoms approach each
other and the time step is long they may "deeply interpenetrate each
other's steeply repulsive wall between steps. The resulting force is
now so large that the atoms fly off at a speed about 1 A/fs (or 10^5
m/s). An equivalent temperature would be in the hundreds of thousands
of degrees".

If a Morse potential is used for the bond-streching term in the
forcefield also the bonded atoms probably behave by this way, because
this potential allows the bonded atoms to drift unrealistically far
apart. If you are using harmonic potentials then I don't know what
happens.

There is some discussion about possible errors in a MD program in the
Section 3.5 "Check on accuracy" (pp. 98-101) in the Allen &
Tildeslay's book "Computer Simulations of Liquids"

Regards,

   Raimo

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