[Wien] query about the C/A optimization of an ABX_3 type halide perovskite in WIEN2k.

[Gavin Abo]

I have some information for your question 1.  If you look at the Fortran 
code in $WIENROOT/SRC_trig/findMINcboa.f, the lattice constants a, b, 
and c in units of bohr are read from calculation directory's init.struct 
file.

The lattice constants are used to calculate a volume in bohr^3. In other 
words:

Conv. Unit Cell Vol = a*b*c

You can also see that the program outputs from that the "Conv. Unit Cell 
Vol" in units of Ang^3.

It looks like there is no code there to detect that you have a hexagonal 
crystal (e.g., with a = b). So, if you want the hexagonal unit cell 
volume [1], you would need to multiply it by the sin(60°):

Hexagonal unit cell volume = sin(60°)*Conv. Unit Cell Vol = 0.866*a*b*c

[1] https://webmineral.com/help/CellDimensions.shtml

Kind Regards,

Gavin
WIEN2k user

On 2/19/2025 5:04 AM, Piyush Kumar Dash wrote:
> Dear Sir,
>
>  I am running wien version 23.2 on a machine of type x86_64 with 
> operating system Cent OS, Kenelversion 3.10.0 - 1160 el7, fortran 
> compiler GNU Fortran version 4.8.5 and math libraries 
> Intel-oneapi-mkl-2021.4.0.
>
> I am working on the C/A optimization of an ABX_3 type halide 
> perovskite using PBE functional. Previously I have performed the 
> volume optimization and from the BM equation of state got the volume 
> V0 = 219.0478 Ang^3 in rhombohedral system. As the hexagonal unit cell 
> volume is 3 times the rhombohedral unit cell volume, the optimized 
> volume is taken to be V = 657.1434 Ang^3 from which the hexagonal 
> lattice parameters are obtained using the initial C/A ratio (=1.2963) 
> to be A = B = 8.3651 Ang and C = 10.8440 Ang, using the formula V = 
> 0.866(A^2)C. Using these inputs, when I performed C/A optimization 
> keeping the volume and B/A constant I got the outputeplot file as 
> given below.
>
> My queries are-
>
> 1) What is this conventional unit cell volume in the .outputeplot file?
>
> 2) When I performed halide variation, the lattice angle in 
> rhombohedral system should follow a trend (I<Br<Cl) but from the above 
> calculations for all three halides the lattice angle order is found to 
> be (I>Cl>Br). Is there any problem in my C/A optimization calculation 
> as the rhombohedral lattice angle depends only on the C/A ratio of the 
> hexagonal system (alpha = 2*sin^(-1)[3/(2sqrt(3+(C/A)^2)]) ?
>
>
>
> outputeplot for ABI_3 generated
>
> ******************************
>  ==========================================================
> Lowest point on X-axis is =  -1.736300
> Minimum value of C/A is =     1.273822
> Minimum value of ENE is =    -52875.865152 Ry
>  ==========================================================
> Value of A is =     15.90048 bohr  ;    8.41417 Ang
> Value of B is =     15.90048 bohr  ;    8.41417 Ang
> Value of C is =     20.25438 bohr  ;   10.71816 Ang
>  ==========================================================
> Initial value of  C/A is =          1.296330
> Conv. Unit Cell Vol =  5120.8186 bohr^3 ;   758.8272  Ang^3
>  ==========================================================
> ******************************