[gmx-users] grompp single vs double precisions
Mark Abraham
Mark.Abraham at anu.edu.au
Sat Mar 4 07:44:05 CET 2006
Yang Ye wrote:
> grompp generates a tpr at the precision that grompp is compiled. My
> concern whether the difference introduced by single-precision tpr has
> made the starting point energy different, especially with automatic LJ
> and pair interaction generation.
Sure. However the total energy function is a fairly slowly varying
function of the positions and velocities; the difference in energy
between one set of generalized coordinates at double precision, and the
energy of the same set truncated to float precision will be very small.
I think the same argument goes for things like LJ parameter generation -
C6(AB) = sqrt( C6(A) * C6(B) ) will differ by only miniscule amounts for
double precision C6(A) and C6(B), and the same values truncated to float
precision. Then, you have to add together a large number of C6 terms in
calculating an energy before such differences propagate meaningfully.
Even if the magnitude of the C6 energy component is around 0.1 kJ/mol,
the maximum difference between the value represented at the two
precisions is going to be 10^-10 kJ/mol or something... you now need to
add N=10^10 or more of those before you'll notice anything - the
assumed-normal estimated error growing with sqrt(N) of course.
I think that only for extremely large systems does this have the
potential to matter much - especially given the general dodginess of the
assumptions underlying MM force fields - and the system of the original
poster was a single molecule in vacuo....
Notwithstanding the above, you could probably construct a pathological
small molecule case where the starting configuration was very near a
transition state with weirdly spiraling valleys where the difference in
precision truncation was enough to make a geometry step move from one
valley to the other...
Mark
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