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

[David van der Spoel]

ms wrote:
> 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
Which gromacs version?

-- 
David van der Spoel, Ph.D., Professor of Biology
Molec. Biophys. group, Dept. of Cell & Molec. Biol., Uppsala University.
Box 596, 75124 Uppsala, Sweden. Phone:	+46184714205. Fax: +4618511755.
spoel at xray.bmc.uu.se	spoel at gromacs.org   http://folding.bmc.uu.se