Dear all, Thanks for all your replies.
I think the difference of 0.0001eV is very big. I wonder is this relevant to the presion setting in the precision.F file? For single precision use a selected_real_kind(6,30) kind and for double precision use a selected_real_kind(14,100) kind. I now do not know the energies are all relevant to the double precision kind data. If they are use some single precision kind data, the difference seems to be understandable because what Ray has said. But if the energies are all relevant to the double precision kind data, if there is a difference on the fourth decimal, it need hundreds of thousand accumulation to achive such a differece. Best regards Guangping 2012-03-25 Guangping Zhang 发件人：Ray Sheppard 发送时间：2012-03-25 09:53 主题：Re: [SIESTA-L] results denpend on the number of core using 收件人："siesta-l"<[email protected]> 抄送： Lets try again. On 3/24/2012 9:43 PM, Ray Sheppard wrote: Hi Guangping, Nick and everyone, I was waiting for someone to write in but I thought I might toss in my two cents. First, in the data you sent, you had about 7 or 8 digits of precision in your calculations. The number system in a computer is a closed set. At some point you go from a very large number to a very small number by adding a positive value. So, computers use approximations by definition. IEEE 754-2008 defines required precision standards. We will skip over the whole, you can not achieve greater precision than your least precise input part and the Numerical Analysis, how these these precisions propagate through a calculation part. A single precision (32-bit floating point number) has about 7 and a quarter digits of precision according to the standard. Beyond that, you will need to understand how error terms propagate as they are combined. A particular processor might have a term E1f. 12*E1f is not the same as the sum of the series X=1->12 EXf where each X is on a unique processor. For numbers of X less than about a thousand, this is usually negligible. In other words, the sum of the differences in the error functions falls at least one digit below the guaranteed precision. So, in short, the fact that your computer will happily print out a bunch of lower significant digits, that does not mean they have any meaning. I hope this is helpful. Ray On 3/24/2012 6:15 PM, Nick Papior Andersen wrote: 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]
