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
<[email protected]
<mailto:[email protected]>> 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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