[Pw_forum] oxygen pseudopotential in tio2

[mohaddeseh abbasnejad]

Dear Dr. Deyu Lu

I am looking for a Ti norm-conserving PSP.
I was wondering if I could have the generated Ti-PSP by you.

Best Regards,
Mohaddeseh
---------------------------------------------------------
Mohaddeseh Abbasnejad,
Room No. 323, Department of Physics,
University of Tehran, North Karegar Ave.,
Tehran, P.O. Box: 14395-547- IRAN
Tel. No.: +98 21 6111 8634  & Fax No.: +98 21 8800 4781
Cellphone: +98 917 731 7514
E-Mail:     m.abbasnejad at gmail.com
Website:  http://physics.ut.ac.ir
---------------------------------------------------------


On Tue, Jun 28, 2011 at 5:57 AM, Deyu Lu <deyulu at yahoo.com> wrote:

> Dear PW developers and users:
>     I have been doing norm-conserving PSP (PBE) calculations to
> study the structure of rutile tio2. During my study, I found the
> resulting lattice constant is sensitive to some fine details of
> the PSPs I used. Such effect is kind of counter-intuitive. Perhaps
> someone with a better understanding can clarify it for me. The story is a
> bit long, which I summarize below. Input files and some PSPs are attached.
>
> All the calculations are carried out on a Linux cluster with QE 4.2.1.
> The code is compiled with Intel compiler (ifort) and mvapich2-1.2,
> and is linked to the mkl library accompanied with the code. Main
> results are verified on a Cray XE6 machine (hopper at NERSC) with the same
> version of QE.
>
> The Ti PBE PSP was generated from an ionic configuration
> 3s2 3p6 3d0 4s0, with semi-core included explicitly. A small rcut
> (0.9 0.9 1.0 for 3s 3p 3d) was used to ensure a good transferability.
> The psp was generated from opium, and the convergence can be
> reached at ecutwfn=160 Ry.
>
> The structure of Rutile TiO2 was calculated using this Ti PSP
> and O.pbe-mt.UPF from QE library. The lattice constant 4.587 angstrom
> is in excellent agreement with experiment (4.587 and 4.582 angstrom)
> but at odds with all the known theoretical results (ultrasoft and paw)  in
> the literature (4.63 - 4.65). The error is about 1.1%. So our good agreement
> seems to be accidental. In order to obtain a better understanding, I
> performed the calculations using paw (vasp) and norm-conserving PSP with HGH
> (abinit library Theor. Chem. Acc. 114, 145 (2005)) which is known to be very
> accurate. Indeed, the results are consistent with literature.
>
> The 1% difference was later found to be related to the Oxygen PSP.
> Different Oxygen PSPs are used in the following tests.
>
> 1.d: 08-O.PBE.fhi.UPF (abinit PSP library, converted to UPF, lmax=3,
> local=2)
>     a=4.633 angstrom
>
> 2.d: O.pbe-mt.UPF (QE PSP library, lmax=2, local=2)
>     a=4.587 angstrom
>
> In order to see if the f channel can cause any difference, 08-O.PBE.fhi.UPF
> is modified (08-O.PBE.fake.UPF) by removing f channel and making
> corresponding changes in DIJ.
> No difference is found as compared to 1.d.
>
> 6.d: 08-O.PBE.fake.UPF (modified version, lmax=2, local=2)
>     a=4.633 angstrom
>
> There seems to be a bug in the <PP_HEADER> section of O.pbe-mt.UPF, where
> Max angular momentum component is set to 1 instead of 2. I manually changed
> it to 2 (O.pbe-mt.9.UPF), and no difference found in output as compared to
> 2.d. So this field is not sensitive in the calculation.
>
> 9.d: O.pbe-mt.9.UPF (minor modification of O.pbe-mt.UPF)
>     a=4.587 angstrom
>
> In the rest of tests, the "Max angular momentum component" is left
> unchanged.
>
> To find out the difference between 08-O.PBE.fake.UPF and O.pbe-mt.UPF, I
> plot term by term for two PSPs, and the results are shown in the xmgrace
> plots for mesh, local, and nonlocal parts of the PSP. The two meshes have
> the same size, with a max difference within 5d-3 a.u (mesh.agr). There is no
> difference found in local and non-local components of the PSP (local.agr and
> nonlocal.agr). In the inset of local.agr, we see that even the small wiggle
> at the origin is id
> entical. My conclusion is that the two PSPs are literally the same.
>
> Further tests were done by replace local PP and Beta sections in
> O.pbe-mt.UPF one by one by the corresponding section from 08-O.PBE.fake.UPF,
> and the difference is negligible as compared to 2.d.
>
> In the end, I replaced </PP_MESH> section in O.pbe-mt.UPF with that from
> 08-O.PBE.fake.UPF, and kept everything else unchanged, the lattice constant
> increased from 4.587 to 4.633.
>
> 8.d: O.pbe-mt.8.UPF ( </PP_MESH> section modified)
>     a=4.633
>
> In the end, I was quite confused about the fact that a very small change in
> the mesh (<5d-3 max at 80 a.u.) can cause a difference in the lattice
> constant by 1%.
>
> Best
> Deyu Lu
>
> ************************************************
> Deyu Lu
> Assistant Physicist, Theory & Computation Group
> the Center for Functional Nanomaterials
> Building 735, Brookhaven National Lab
> Upton, NY, 11973
> ************************************************
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