As well as reductions in parallel should never be expected to yield the exact same result every time. There are numerical rounding differences when not enforcing the same reduction scheme every time, which is the case when using MPI-library reductions. I will not argue whether it can or can not contribute to an error of that order, but the fact that your convergence path is not similar I would not expect the exact same result, after all you are targeting a tolerance, not an "exact" solution.
But also as Huan notes, is it the same executable? Nick 2012/3/24 Huan Tran <[email protected]> > I think 0.001 eV is a small quantity, and it may come from different > library when you run 1 core (serial) and 12 core (parallel). > > > > > On Sat, Mar 24, 2012 at 5:50 AM, Guangping Zhang <[email protected]> wrote: > >> Dear siesta users: >> >> I have now encounter a question: the results differ when using different >> number of core to parallel. >> >> 1 core: >> siesta: 124 -33928.6003 -33928.6031 -33928.6366 0.0002 -4.1261 >> siesta: 125 -33928.6004 -33928.6012 -33928.6346 0.0001 -4.1251 >> siesta: 126 -33928.6003 -33928.6009 -33928.6344 0.0001 -4.1258 >> >> siesta: E_KS(eV) = -33928.6012 >> >> siesta: E_KS - E_eggbox = -33928.6012 >> >> 12 core: >> >> siesta: 123 -33928.6004 -33928.6064 -33928.6398 0.0003 -4.1246 >> siesta: 124 -33928.6003 -33928.6026 -33928.6360 0.0001 -4.1257 >> siesta: 125 -33928.6003 -33928.6018 -33928.6352 0.0001 -4.1256 >> siesta: 126 -33928.6004 -33928.5996 -33928.6331 0.0001 -4.1253 >> >> siesta: E_KS(eV) = -33928.6001 >> >> siesta: E_KS - E_eggbox = -33928.6001 >> >> The E_KS differs 0.001 eV. I think it is significant. I think the >> number of core using to paralle only affect the speed. The results should >> be the same. Or there is something I am missing? >> >> And I have another question: when I do the geometry optimization, which >> energy is used to calculated the force? And when I do molecular dynamics, >> which energy is used for the force? >> >> I think the KS energy of a single point DFT calculation and the KS >> energy of a molecular dynamics should be the same for the same structure. >> Am I right? >> >> Any suggestions will be regarded. >> >> Best >> >> Guangping >> >> 2012-03-24 >> ------------------------------ >> Guangping Zhang >> > > > > -- > Regards > Huan Tran > Department of Physics > University of Basel, Switzerland > Email: [email protected] >
