I do not have any reference and while writing I actually had in mind quantum zero-point effects, which admittedly have strictly not much to do with the breakdown of the the BO approximation. I would’t be surprised, though, if some H-rich small-gap system would show departures from the BO picture. Hope this clarifies. SB
> On 10 Jan 2019, at 14:54, Robert Molt <[email protected]> > wrote: > > 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] >> >> >> >> >> >> >> >> _______________________________________________ >> users mailing list >> [email protected] <mailto:[email protected]> >> https://lists.quantum-espresso.org/mailman/listinfo/users >> <https://lists.quantum-espresso.org/mailman/listinfo/users>-- > Dr. Robert Molt Jr. > [email protected] > <mailto:[email protected]>_______________________________________________ > users mailing list > [email protected] > https://lists.quantum-espresso.org/mailman/listinfo/users — Stefano Baroni - Trieste — http://stefano.baroni.me <http://stefano.baroni.me/>
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