[Pw_forum] Irreps and vibrational modes

[Elie M]

Dear all,
I am calculating the phonon modes at Gamma=0 for a 42 Carbon atom supercell. If  N=42 then one has 3N= 3*42 modes= 128 modes. Just to confirm:
(1) Are all the modes vibrational?
(2) Are any modes missing from the calculation like optical or accoustical modes?
(3) i did the calculation and got two of the modes negative which means either threshold is high or the ecutwav must be at least eight times ecutrho. Is a threshold of 10^(-15) still too high for such a system?

Thanks
ElieUniversity of NottinghamNG7 2RDUK 		 	   		  
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