[Wien] proper k-points for Nd

Victor Zenou zanov at post.bgu.ac.il
Fri Jan 18 12:33:12 CET 2019


Hi LDM

The unit cell of Nd is hexagonal, space group *P63/mmc*  (#194), Pearson
Symbol: hP4 with lattice parameters: a= 3.659(3), c= 3.659(3) according to
B.J.Beaudry, P.E. Palmer, "The Lattice parameters of La, Ce, Pr, Nd, Sm, Eu
AND Yb", J. Less-Common Metals, 34 (1974) 225 - 231.

The interatomic distances are 3.628A to 3.659A. So yes, these are large
distances.

I can use smaller RMT for Nd and/or HDLOs.

Why do I need the same RMT of element when comparing different phases with
that element?

Victor



‫בתאריך יום ה׳, 17 בינו׳ 2019 ב-23:58 מאת ‪Laurence Marks‬‏ <‪
L-marks at northwestern.edu‬‏>:‬

> Dear Viktor,
>
> Let me add something about large RMTs. I have noticed in the past that, in
> general,  the gradient at the RMT calculated from both sides (x RMTCheck)
> is very similar, e.g.
>
> Atom   1 O    | RMT Charge   0.489 Grad   0.690 | Step Charge  0.00136,
> 0.00000 Gradient   0.2655, -0.2655 O
>
> The last two numbers should sum to almost zero. However, with very large
> RMTs they can deviate from this, particularly when the gradient is small. I
> think this leads to anomalies at the RMT. (Peter would say that the
> linearization is breaking down with an inadequate basis set, or something
> similar.) Reducing the RMTs generally avoids this.
>
> While it is true that larger RMTs are "better" in terms of speed for a
> single iteration, slightly smaller RMTs which may take longer for a single
> iteration but are more stable so require fewer iterations can be nett
> faster. Speed is not the same as stability and convergence, and often they
> oppose each other. (Think about the tradeoff with condenser aperture size:
> smaller is more coherent but more drift...)
>
> N.B., are you really sure that the Nd distances are that large?
>
> On Thu, Jan 17, 2019 at 3:31 PM Peter Blaha <pblaha at theochem.tuwien.ac.at>
> wrote:
>
>> RMTs:  Make sure that you choose RMTs, which can be used in all
>> elemental solids, but the same should also be used in your compound.
>>
>> Yes, you can use very large RMTs if you follow the recommendations
>> during init_lapw:
>> Use HDLOs for f and (as L.Marks mentioned) probably d.
>> Use lvns=6 (or 8) in case.in1
>>
>> For a difficult system (and Nd with partially occupied 5f electrons is
>> definitely a difficult systems), scf convergence can be more difficult.
>> Thus, as mentioned before, use both -ec and -cc (at least 0.001;
>> eventually a better cc is difficult to reach. Often you may need MORE
>> than the default max of 40 scf cycles. So when runsp_lapw stops, make
>> sure it does not stop beucause of the 40 it limits.
>> Clearly, your energies with :dis=0.2 are wrong.
>>
>> For the compound but also the elements, make sure to use "consistent
>> RKMAX" values.
>> If you use: Al Ti Nd (2.3,2.5,2.7) and for your compound you use
>> RKMAX=8, than   Al should be done with RKMAX=8, Ti with RKmax=8*2.5/2.3,
>> Nd with 8*2.7/2.3
>>
>> Am 17.01.2019 um 21:19 schrieb Victor Zenou:
>> > Dear Lauri
>> > Thanks for your answer
>> > I used only energy convergence criteria (10^-4). Still the charge
>> > convergence was between 0.006 and 0.2 e.
>> > The nearest neighbor distance is 6.856 a.u, I used RMT=2.7 a.u., summed
>> > to 5.4 which is much lower than 6.856. Usually large RMT is preferred
>> > to make the calculations as fast as possible. I plan to check a phases
>> > in Al-Ti-Nd and I chose RMT equal to 2.3, 2.5 and 2.7 respectively.
>> > Victor
>> >
>> >
>> >
>> > ‫בתאריך יום ה׳, 17 בינו׳ 2019 ב-11:16 מאת ‪Laurence Marks‬‏
>> > <‪L-marks at northwestern.edu <mailto:L-marks at northwestern.edu>‬‏>:‬
>> >
>> >     With such large RMT you certainly need HDLO for Nd, perhaps both d &
>> >     f. I would not want to use such large RMTs.
>> >
>> >     Have you checked that the charge convergence is good?
>> >
>> >     _____
>> >     Professor Laurence Marks
>> >     "Research is to see what everybody else has seen, and to think what
>> >     nobody else has thought", Albert Szent-Gyorgi
>> >     http://www.numis.northwestern.edu <
>> http://www.numis.northwestern.edu>
>> >
>> >     On Thu, Jan 17, 2019, 03:04 Victor Zenou <zanov at post.bgu.ac.il
>> >     <mailto:zanov at post.bgu.ac.il> wrote:
>> >
>> >         Hi
>> >         Here is updated question:
>> >
>> >         Dear Wien2k users
>> >
>> >         I'm using wien2k version 17.1 installed on Ubunto 18.04.
>> >
>> >         I need to calculate the enthalpy of formation of few
>> >         intermetallic phases that include Nd. For that element I used
>> >         RMT=2.7 a.u. and RMT*Kmax=8.5 for spin-polarization calculations
>> >         via GGA-PBE. I didn't try spin-orbit coupling. Yet.
>> >
>> >         The first stage is to choose proper k-points, which will give
>> >         accurate results (I used 1x10^-4 Ryd for energy convergence) on
>> >         one hand, but on the other hand won't be expensive in terms of
>> >         computing time.
>> >
>> >         I noticed that there are large energy fluctuations (1x10^-2 Ryd;
>> >         which are 2 order of magnitude higher than the energy accuracy
>> >         (defined by convergence criterion), even when I went to 15,000
>> >         k-points. Here is a list of energy as function of k-points:
>> >
>> >         K-points          IBZ      E [Ryd]
>> >
>> >           100       8        -77040.4692
>> >
>> >           500     38        -77040.4780
>> >
>> >         1000    81        -77040.5062
>> >
>> >         2000    120      -77040.5061
>> >
>> >         3000    208      -77040.4391
>> >
>> >         4000    244      -77040.4699
>> >
>> >         5000    280      -77040.4878
>> >
>> >         7500    455      -77040.4707
>> >
>> >         10000  540      -77040.4881
>> >
>> >         15000  840      -77040.4694
>> >
>> >         I would be happy to get an idea what could have gone wrong.
>> >
>> >         Best regards, Victor
>> >
>> >
>> >
>> >         ‫בתאריך יום ד׳, 16 בינו׳ 2019 ב-13:59 מאת ‪Victor Zenou‬‏
>> >         <‪zanov at post.bgu.ac.il <mailto:zanov at post.bgu.ac.il>‬‏>:‬
>> >
>> >             Dear Wien2k users
>> >
>> >             I'm using wien2k version 17.1 installed on Ubunto 18.04.
>> >
>> >             As a part of investigation of intermetallic phases in
>> >             Al-Ti-Nd system, I need to calculate the enthalpy of
>> >             formation of few phases in that system. For that purpose, I
>> >             start calculating enthalpy (equal to energy at 0K) for each
>> >             element and phase at their optimal relaxed state. I used
>> >             spin-polarization calculations. (GGA-PBE) As advised in
>> >             Wien2k website I used the same RMT for each elements: 2.3,
>> >             2.5 and 2.7 a.u. for Al, Ti and Nd, respectively. Also
>> >             proper RMT*Kmax (7.5 to Ti, 8.5 to Nd and 7.0 for the rest).
>> >
>> >             The first stage is to choose proper k-points, which give you
>> >             accurate results (I used 1x10^-4 Ryd for energy convergence)
>> >             on one hand, but also some save computing time.
>> >
>> >             The calculation for all elements and phases went OK, but Nd.
>> >             From certain k-points the energy fluctuated between 2 values
>> >             (differed by 1x10^-2). Attach a list of energy as function
>> >             of k-points:
>> >
>> >             K-pointsIBZ E [Ryd]
>> >
>> >             1008-77040.4692
>> >
>> >             50038-77040.4780
>> >
>> >             100081-77040.5062
>> >
>> >             2000120-77040.5061
>> >
>> >             3000208-77040.4391
>> >
>> >             4000244-77040.4699
>> >
>> >             5000280-77040.4878
>> >
>> >             7500455-77040.4707
>> >
>> >             10000540-77040.4881
>> >
>> >             15000840-77040.4694
>> >
>> >             I tried to get smart and did volume optimization using 4000
>> >             k-points, later 5000 k-points. I got strange results.
>> >
>> >             I would be happy to get an idea what could have gone wrong.
>> >
>> >             I didn't try spin-orbit coupling
>> >
>> >             Best regards, Victor
>> >
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>
>
> --
> Professor Laurence Marks
> "Research is to see what everybody else has seen, and to think what nobody
> else has thought", Albert Szent-Gyorgi
> www.numis.northwestern.edu ; Corrosion in 4D:
> MURI4D.numis.northwestern.edu
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