[gmx-users] problem of tabulated bonded potential

[Z.Xiao]

Dear all gmxers,
I meet some problems when I use the tabulated bonded potential.
My original function is a sum of An*cos(x)^n (n=1-8).For short here I replaced it by cos(x).
if f(x)=cos(x),then -f'(x)=sin(x).and the numberical derivative -f'(x)=-(y(n+1)-y(n-1))/2.
but there are great discrepancy between the two -f'(x).
which is right?
and if mdrun with the derivative of original function I would met a warning:the forces deviate
on average 207% from minus the numerical derivative of the potential.but mdrun with the the
numberical derivative there is no warning.
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