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

[Piyush Kumar Dash]

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
 ==========================================================
******************************
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