The elastic constant matrix in setting.yaml

AMSET
1

Dear developer,

I wonder what is the input format of elastic constant matrix in settings.yaml file?
I checked the manual in the website and example files.

The unit of these elastic constants are not mentioned, I guess maybe in GPa (it’s kbar in OUTCAR, but usually we find GPa in papers).

The index issue.
In VASP, the format in OUTCAR is like :
TOTAL ELASTIC MODULI (kBar)
Direction XX YY ZZ XY YZ ZX
XX … … … … … …
YY … … … … … …
ZZ … … … … … …
XY … … … … … …
YZ … … … … … …
ZX … … … … … …

Found in wiki, the stiffness tensor C_ij, C_44 is for YZ, C_55 for XZ, C_66 for XY.
The C_44,C_55,C_66 are at different positions.

Not sure AMSET follows which form?

2

Hi John,

Thanks for pointing this out. I hadn’t realised the documentation was unclear.

The unit is GPa, so you will need to convert the VASP output from kbar.

The format is the standard Voigt tensor notation. E.g., see this image taken from: Introduction to Elasticity/Constitutive relations - Wikiversity

926c28be43c36ca01494fd55cac9ee8d4d43ce7d|0x0

This means the VASP output will need to be reordered a little. I will hopefully release a command line too to do this soon.

Best,
Alex

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3

Thank you! Alex.
I finally figure out this.
Maybe add this detail to the manual webpage is a simply way.
Otherwise, I prefer to write a simple script to read elastic and dielectric matrix from OUTCAR from VASP calculations (fixed name like OUTCAR_elast and OUTCAR_dielec), then maybe print to settings.yaml.

Best,
John Chen

4

FYI, I made a simple command line script using awk that will extract the elastic constant, make the unit conversion and reorder the tensor. I will add this to amset soon, but in the time being it is available here:

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