Hi, I've come across a discussion about the "single X double precision" issue in a NAMD users list e-mail ( http://www.ks.uiuc.edu/Research/namd/mailing_list/namd-l/9166.html). I would like to know what do you think about these 3 specific points:
1) "there are some places where single precision (FFT, force/energy computation), where single precision can be applied, but in other places (summations) where even double precision may result in artefacts, due to limited numerical accuracy for large enough systems. there are some applications that show little sensitivity to single/double precision issues (homogeneous bulk lennard-jones systems), but others are notoriously difficult (systems with significant potential drops in one or two dimensions, e.g. metal surface slabs or lipid bilayers) since you don't have that much error cancellation anymore." 2) "if you store coordinates in single precision, you have to take extra precautions for handling very large systems, since you would store coordinates with different relative precision, depending on how close you are to the origin. e.g., with domain decomposition, you can define per domain offsets, but that would work only well in case of a large enough number of domains. " 3) "the best way to study the impact of single vs. double would be by running tests with the gromacs code. gromacs _can_ be compiled in single or double precision. for a proper comparison, you'll have to turn off the assembly innerloops though, since they may use single precision even in double precision mode." Specifically this last point startled me. In fact, is this true? Cheers, Pedro.
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