[gmx-developers] single precision, double precision hybrid GROMACS

D. Ensign densign at stanford.edu
Fri Apr 7 21:28:20 CEST 2006

says Eric Lindahl:
> I think the only case for "limited double precision" is to get
> perfect energy conservation with constraints.

Hold on, let's stop right there for a minute. Not that this train has already passed by.

Allow me to re-muddy the waters after Michael's clarification. Please don't let it matter
to you WHY I want to do certain things in double precision and others in single --
although it's not a big mystery, perhaps we can all pretend that I'm doing it because of
the voices in my head, and what's a better reason than that? Besides, someday I'm sure
I'll see you guys at a meeting, and I'll be jabbering on about what a great idea it was
to do some calculations in single prec. and others in double. Then you'll all make fun of
me and poke me with sharp sticks, until I start crying and run away to hide in the
cupboard. I will not be coaxed out except with Gouda, Dutch beer, and gentle coos of
"That's a nice little monkey, that's a boy." All in good time, my friends, all in good

That being said, what would be easier to appease the voices:
1. Use the single precision code and make the constraints double
2. Use the double precision code and make the forces single (the force calculations look
like they're all OVER the place, but I bet I can track them all down)
3. Compile single precision and double precision libraries, then link, eg, single
precision fnbf.o with, eg, double precision md.o


Dan Ensign
Stanford University
"Give up learning, and put an end to your troubles." -- Lao Tzu

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