[Pw_forum] Re: About: the system is metallic, specify occupations?

Guido Fratesi guido.fratesi at unimi.it
Wed Nov 22 09:59:30 CET 2006

>    H      0.000000000    0.000000000    3.250000000
>    C      0.000000000    0.000000000    0.000000000
>    C      0.333333333    0.666666666    0.000000000
>    C      0.000000000    0.000000000    0.500000000

You have an odd number N of electrons per unit cell, so your system IS
metallic, and one cannot simply occupy the lowest N/2 bands as would be 
the default for an insulator.
Hence you have to specify how to fill the electron bands.
See file Doc/INPUT_PW, variables "occupation", "degauss", "smearing" and 
references given there for details.

[If you want to think more about that, consider an H atom in a large box, 
suitable for an isolated-atom calculation. Then you have 1 electron per 
unit cell, which is odd again, so the system made of the repeated H 
atoms far away the one from the other is metallic, and you will find 
with PWscf that the lowest energy is given by half-filling a band which 
is nothing more than the (bonding) combination of the 1s orbitals, as it 
should be. Moreover, the results will not be much affected by the values 
you choose for the variables specified above. Can you imagine why?]

Hope this helps,

Guido Fratesi
Dipartimento di Fisica
Universita` degli studi di Milano
Via Celoria, 16 - 20133 MI (Italy)

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