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Dear Axel,

Thank you for your answer. I have run the program with run-style verlet. It
works for a short time step of 0.05 fs. I Think this sjort time step is
necessary because of the bond vibration.

that is very hard to believe. are you sure there is nothing wrong with
your force field parameters. for simple molecular systems, you can
usually run with up to 0.2fs time step. the fact that you see the
system collapse would hint at an issue with the non-bonded parameters,
but that is very difficult to say in this generality and without
knowing more details.