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<TITLE>Re: [gmx-developers] double precision minimization accuracy limits.</TITLE>
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<P><FONT SIZE=2>> <BR>
>Unless I'm missing something, this is a users-list question. Energy <BR>
>minimization is going to get stuck in a local minimum.<BR>
><BR>
But the local minimum should be a minimum down to machine accuracy - at least thats<BR>
achievable with force field/MD programs I have written and used in the past - unless there are problems with<BR>
derivatives due to discontinuities (hence the use of switching functions) or derivative errors (ie. there are numerical errors in the<BR>
derivatives which are due to programming errors or approximations used in computing the forces<BR>
from the coordinates and their correspondence to the energies).<BR>
Hence the developers list - I dont know how most users are going to know the exact numerical coding<BR>
of the derivative computation functions.<BR>
<BR>
>The acid-test of <BR>
>a minimization is if the resulting structure can be used to start an MD <BR>
>run.<BR>
Not true - energy minimization is required for example to compute normal modes<BR>
where good convergence is needed for isolated molecules to ensure the 6 lowest frequency<BR>
modes are close to zero - such structures need to be far more converged (ie. the maximum derivative<BR>
should be less than 0.10 kJ nm-1 - we used to use < 0.001 kcal angstrom-1)<BR>
to produce valid results whereas MD can be started from much larger derivatives.<BR>
<BR>
To me a maximum derivative of 232 kJ nm-1 is too big - a double precision program<BR>
should be able to get them lower than that even for large protein systems.<BR>
<BR>
The definition of a local minimum is all derivatives are zero<BR>
to infinite precision - otherwise its not a local minimum or minimum of any kind.<BR>
For limited precision computers we accept a definition of say the maximum derivative < 0.01 kJ nm-1.<BR>
<BR>
<BR>
>I don't see why you couldn't grompp your next energy minimization using <BR>
>-t .trr -e .edr but again, that should not be necessary. <BR>
<BR>
thanks - I missed that possibility.<BR>
<BR>
David </FONT></P>
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