I am struggling to find an explanation for the difference between a spectral band structure task and a normal band structure task. I have attached example .param files. Also, does the BS_XC_FUNCTIONAL have the same implementation as the SPECTRAL_XC_FUNCTIONAL parameters?Spectral Bandstructure .param file [Normal CASTEP Bandstructure .param file]2

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These input param files both compute the same thing (the bandstructure or DOS, depending on the k-point set you provide) but the "spectral" route uses the newer spectral module, which has a more robust and efficient optimisation algorithm.

In older versions of CASTEP the spectral module did not support all of the functionality of the bandstructure module, which is why both methods were supported. I'm not aware of any missing functionality in the spectral module now, and at some point in the future the old bandstructure module will be retired, and

task : bandstructure

will be updated to just be an alias for

task : spectral
spectral_task : bandstructure

(the spectral_task keyword is mostly to tell CASTEP not to bother computing things like optical matrix elements).

There is one thing you have to be careful with, which is that a spectral calculation will read the spectral kpoint keywords from the cell file (spectral_kpoint_path etc) whereas a bandstructure calculation will read the bandstructure k-point keywords (bs_kpoint_path etc).

As for the exchange-correlation functional keywords, I think the implementation is identical.


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