[QE-users] differently complied QE

[Robert Molt]

Question:

You mentioned the BO as being a limitation on the accuracy of lattice 
constants. Can you cite a reference on this for reading? I was unaware 
that this was ever meaningfully a source of error in an equilibrium 
geometric property.

IUPUI

On 1/7/19 10:54 PM, Stefano Baroni wrote:
> Aleksandra:
>
> you are very right and I commend you for your thoroughness. Only 
> figures that can be reproduced should be reported. Usually, lattice 
> constants can be computed and reported with a higher accuracy than you 
> observe. This being said, as Nicola says, the *numerical* accuracy 
> that can be achieved is usually higher than needed for practical 
> purposes and higher than the accuracy due to “unavoidable” physical 
> approximations (first and foremost the XC functional, but also more 
> fundamental ones, such as Born-Oppenheimer’s).
>
> Coming to the point, that numerical results may depend on such 
> seemingly irrelevant “details" as hardware, mathematical libraries, 
> optimisation level, etc. is not uncommon, as surprising as it may 
> seem, particularly on parallel machines. The result of many 
> “commutative” operations (sums, products) depends in practice on the 
> order in which these operations are performed, due to the 
> non-commutativity fo rounding errors, and this order does depend on 
> all those “details”. Some properties may be more sensitive to these 
> details than others in some systems more than in others. For instance, 
> structural parameters (such as lattice parameters or atomic positions) 
> are more sensitive to the accuracy in stresses and forces in soft 
> systems than in hard ones. Can you tell why?
>
> Bottom line. Please, do the tests that Nicola suggests and make sure 
> you understand the results you get (I am sure you are able to 
> appreciate if and when you understand, which is not so common). In 
> case of need, revert to us.
>
> Good luck!
> Stefano Baroni
>
>> On 8 Jan 2019, at 00:49, Aleksandra Oranskaia 
>> <aleksandra.oranskaia at kaust.edu.sa 
>> <mailto:aleksandra.oranskaia at kaust.edu.sa>> wrote:
>>
>> Thank you for the respond!
>>
>> I was confused with the difference, because if the 3rd digit is 
>> “arbitrary” (depends on libraries, compilers, hardware, etc.) than 
>> there is no sense in reporting lattice constants in papers with this 
>> accuracy; however, it is a common practice.
>>
>> I used:
>> conv_thr=1.d-8
>> tot_conv_thr=1.d-6
>> forc_conv_thr=1.d-6
>> cell_dofree=‘volume’ (for the cubic cell)
>>
>> I will try to increase conv_thr and press_conv_thr, as well as to 
>> compare gnu and intel compillations, and reply back.
>>
>>
>> Best,
>> Alex.
>>
>> ___
>> Aleksandra Oranskaia (M.Sc.)
>> ChemS PhD student, KAUST
>> Phone: +966 50 1335254
>
> —
> Stefano Baroni -  SISSA, Trieste - http://stefano.baroni.me 
> <http://stefano.baroni.me/>, stefanobaroni (Skype)
>
> And aren’t we all tired of those who claim to know the answer to life, 
> death and the creation being so […] sensitive about their knowledge? 
> [Stephen Fry]
>
>
>
>
>
>
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-- 
Dr. Robert Molt Jr.
r.molt.chemical.physics at gmail.com

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