[Wien] accuracy problems

[Peter Blaha]

> We are seeing some accuracy trouble with Wien2k when combining various
> compilers and blas+lapack libraries on PowerMac systems.
> The data from the TiC sample case is as follows:
>
>     compiler      library                                        result
> 1) Absoft         Absoft                                        correct
> 2) Absoft         vecLib                                        incorrect
> 3) xlf               veclib                                         incorrect
> 4) xlf               manually compiled blas/lapack     incorrect
>
> The incorrect results are pretty similar for the three wrong cases. From
> 1-3 the vecLib libraries would seem the culprit. But 4 kills that logic.
>  From this data, it seems that it is not just the compiler or just the
> library. 2+3 tells you it is not just the compiler that is giving the
> error. 3+4 tells you the library is not the sole culprit.
> Someone who was helpful in finding out the above data suggested that it
> could be some numerical instability issue in the Wien2k code. The code runs
> fine of course on many platforms, including PowerMacs using all-Absoft. Has
> anyone ever seen something similar, perhaps using development-stage
> versions of the code?

You did not tell what means: "incorrect" ?

Anyway, I would claim it is due to compiler bugs (+veclib bugs?). At least the
xlf is known for various bugs at some optimization levels.

I'd expect correct behaviour if you explicitly reduce the optimization
(maybe down to -O0).

                                      P.Blaha
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Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
Phone: +43-1-58801-15671             FAX: +43-1-58801-15698
Email: blaha at theochem.tuwien.ac.at    WWW: http://info.tuwien.ac.at/theochem/
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