# [gmx-users] Tabulated potential, derivative errors and interpolation

```Hi,

I am trying to use a tabulated potential but I have a hard time getting
the derivatives right (apparently), which puzzles me.```
```
I tried at first to use the analytical expression for the derivative,
and I got warnings ranging from:

"WARNING: For the 2168 non-zero entries for table 1 in
table_10_70_0.4_5_C-alpha_C-alpha.xvg the forces deviate on average 133%
from minu
s the numerical derivative of the potential

WARNING: For the 257 non-zero entries for table 2 in
table_10_70_0.4_5_C-alpha_C-alpha.xvg the forces deviate on average 300%
from minus
the numerical derivative of the potential"

to

"WARNING: For the 268 non-zero entries for table 2 in
table_10_70_0.4_1_C-alpha_C-alpha.xvg the forces deviate on average
120138% from minus the numerical derivative of the potential"

At first I thought it was because I am artificially "cutting" the
highest values of the repulsive term, putting everything above 10^17 as
10^17 with a derivative of 0, but playing with that didn't help; also I
found that there are errors too in the repulsive part, which hasn't
out-of-range values.

So I thought it could have been some mysterious error in the math, and
tried estimating it numerically, but again with a numerically-estimated
table I have errors, and even worse ones:

"WARNING: For the 257 non-zero entries for table 2 in
table_10_70_0.4_5.xvg the forces deviate on average 120773% from minus
the numerical derivative of the potential"

I used mdrun with -debug 1 to see what tables the guy is actually using,
and I am also concerned, because there is something weird which could
explain the problem: why are *positive* values routinely interspersed
with the right *negative* values in the derivative?

For example, this is an excerpt of rtab.xvg:

3.0992913246e-01  2.7511889648e+02  7.0945126953e+03
3.1012925506e-01  2.7884115601e+02  -1.2228130859e+04
3.1032931805e-01  2.7019714355e+02  -6.7522789062e+04
3.1052938104e-01  2.5448614502e+02  -8.2872140625e+04
3.1072947383e-01  2.3970016479e+02  -5.8276152344e+04
3.1092956662e-01  2.3383134460e+02  6.2780151367e+03
3.1112965941e-01  2.3702958679e+02  -1.0651265625e+04
3.1132972240e-01  2.2957331848e+02  -5.8167523438e+04
3.1152981520e-01  2.1604493713e+02  -7.1314695312e+04
3.1172990799e-01  2.0332643127e+02  -5.0073011719e+04
3.1192997098e-01  1.9829878235e+02  5.5352783203e+03
3.1213006377e-01  2.0104492188e+02  -9.2590859375e+03
3.1233015656e-01  1.9461547852e+02  -5.0073472656e+04
3.1253021955e-01  1.8298132324e+02  -6.1317703125e+04
3.1273031235e-01  1.7205114746e+02  -4.2998656250e+04
3.1293040514e-01  1.6774539185e+02  4.8916777344e+03
3.1313049793e-01  1.7010066223e+02  -8.0468198242e+03
3.1333056092e-01  1.6456481934e+02  -4.3058406250e+04
3.1353065372e-01  1.5456475830e+02  -5.2672746094e+04
3.1373074651e-01  1.4518064880e+02  -3.6885503906e+04
3.1393080950e-01  1.4149731445e+02  4.2984921875e+03
3.1413090229e-01  1.4351583862e+02  -6.9780966797e+03
3.1433099508e-01  1.3875308228e+02  -3.6990878906e+04
3.1453105807e-01  1.3016751099e+02  -4.5200566406e+04
3.1473115087e-01  1.2211988831e+02  -3.1613373047e+04
3.1493124366e-01  1.1897129822e+02  3.7776948242e+03

Thank you for help!
Massimo
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