[gmx-users] Forces vector problem

[Alexander Yakovenko]

Hi All.
I have rather fundamental question that can be addressed to nearly all force field based energy minimization routines. Why do the minimizer can`t go down if the force vector exists? I can realize this problem due to machine precision, but some systems I have met consists of several tens of atoms only and have derivative vector that is incomparable with machine precision level. If we assume that the partial derivatives of elementary energy function are calculated correctly (with incomparable small machine errors) and their number is relatively small (machine_errors*function_number << gradient_lenght), what would be the origin of the problem?
Thank you a lot,
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Alexander Yakovenko
Zabolotnogo str 150
Institute of Molecular Biology and Genetics
Kiev
Ukraine
email: yakovenko_a at ukr.net