XAvier, thanks for your reply. the numberical derivative -f'(n)=dy/dx=-(y(n+1)-y(n-1))/(n+1)-(n-1). so -f'(n)=-(y(n+1)-y(n-1))/2. Is it wrong?
On Nov 9, 2010, at 7:12 AM, Z.Xiao wrote: > 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. here you should have dy/dx, re you really doing this? > 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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