[Wien] band structure dependence on symmetry and complex calculation

[Peter Blaha]

This is related to what is called "backfolding".

Remeber basic solid state theory:

A direct lattice with lattice constant a, has a reciprocal lattice with  b=2pi/a

Now double the direct lattice (a'=2a), the reciprocal lattice b'=2pi/a'= pi/a

Now now "pi/a" referes to the "X" point in the original lattice, but this same pi/a
is a full reciprocal lattice vector for the supercell and thus appears at Gamma.

This is called "backfolding" of of the second half of the Gamma-Delta-X branch back to
Gamma-Delta(now X')-Gamma.

There fore the original Gamma-X gap is now a direct Gamma-Gamma gap.

Am 28.06.2013 19:18, schrieb venkatesh chandragiri:
>
> Dear wein2k users,
>
> I have calculated the band structure of the Fe2VAl lattice (fm3m, 225) for the two cases.
>
> *case one* : using the space group (225) defined structure with three atoms.
>
> *case two* : by seeing the structure of Fe2VAl in xcrysden, i have generated the case.struct file with 16 atomic unit cell (8-Fe, 4-V, 4-Al) with a 16 number of
> co-ordinates such that the structure looks same as in *case one*. But, in this structure file, i have used space group as a primitive (1).
>
> Using the Powder cell software, i also checked the XRD of the structures from the both cases and similar diffraction pattern were found. I run the volume optimization and
> min positions before running the spin polarized SCF calculation. However, In the First case (space group structure), we run the calculation without complex (no inversion).
> But, in the second case (primitive), we did the calculations with keeping complex ON.
>
> Now, i done the spaghetti plots for these two structures. For the space group case, we have observed indirect band picture between gamma and X k-points (X point will always
> followed by gamma). This nature is already reported in the literature also. But, for the second case with primitive structure, i have found the direct band like picture in
> which both band are touching at the gamma point.
>
> In preparing the case.klist and case.klist_band, i have used similar k-point co-ordinates for the both cases.
>
> Why the bands above the Fermi level would not shift to x point in the primitive case. Is there any effect of removing the symmetry on the band structure even though we did
> calculations on the similar structure...?
>
> or else in the first case, calculations did with complex OFF while in the second case with complex ON. Is this difference will change the band structure nature...?
>
> I could not attach the obtained figures, because i sent same mail with figures in few days before but it is not published in wien2k mailing list due to limitations in the
> mail content size.
>
> thanks in advance and waiting for your replies.
>
> regards,
>
> Ch. Venkatesh,
> C/o. Prof. V. Srinivas & Dr. S. K. Srivastava
> Department of Physics & Meteorology,
> IIT Kharagpur.
> ph: +91-3222-283848 (Lab)
>        +919445909693
>
>
>
>
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-- 
-----------------------------------------
Peter Blaha
Inst. Materials Chemistry, TU Vienna
Getreidemarkt 9, A-1060 Vienna, Austria
Tel: +43-1-5880115671
Fax: +43-1-5880115698
email: pblaha at theochem.tuwien.ac.at
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