[Pw_forum] Defining exchange correlation functional

[Kayahan Saritas]

Dear PWSCF users,

For my calculations, I would like to use B3PW91 xc-functional, but it is not in the pre-defined list in the espresso/modules/funct.90 file. However, as it seems from that file, we should be able to use any non-conflicting combination. As we know for B3LYP, 

exchange: 0.8 LDA 0.2 HF (b3lp)
correlation: VWN (wvn)
exchange-correction=0.72 Becke88 (b3lp)
correlation-correction=0.81 LYP (b3lp)

Therefore in input file, dft_input=‘B3LYP' can also be written as  dft_input="b3lp+vwn+b3lp+b3lp”. It is ok until here. 

For B3PW91, three parameters above are the same (see Becke,  JCP 98, 5648 (1993); doi: 10.1063/1.464913 ) but when I write dft_input="b3lp+vwn+b3lp+ggc” (ggc for gradient corrections of PW91) I get an error saying dft_input is conflicting. 

I think I need to use 0.81*ggc actually to represent the B3PW91, but to me it doesn’t seem possible to modify it without accessing to the private variables. 

I would be happy if someone could explain how to make a user defined xc-functional in PWSCF or how the algorithm decides if the case is conflicting or not.

Thanks,
Kayahan