[Pw_forum] time-reversal symmetry broken

[Timo Thonhauser]

Thanks Stefano, Nicola,

I already use the no_sym flag, which prevents the charge density from
being symmetrized with respect to all symmetry operations. However,
even with the no_sym flag, only half the k-points are generated (e.g.
for a 4x4x4 mesh the code generates 32 points and all the ones with
negative k_x are missing). Here Stefano's idea of supplying a k-point
list myself, would help. But, it is not that simple, is it? If inversion
symmetry and time reversal are present, the wave functions can be
chosen real (by real I mean that the wave functions only have a constant
arbitrary phase factor). So, if I break time reversal and give the
code my own, complete k-point list, does the code anywhere
still assume that the wave functions are "real"? Maybe breaking
inversion symmetry by hand could help?

Timo


Nicola Marzari wrote:
> Stefano Baroni wrote:
>   
>> Timo: I do not have a solution handy, and others may have deeper ideas. 
>> I think the problem is that when simmetrizing the charge density the 
>> code always assumes e(k)=e(-k) and psi(-k)=psi(k)*, and so it never uses 
>> more than 1/2 the BZ. One thing you may try is to pass explicitly all 
>> the k-points, and wish that the code is not so smart as to remove 1/2 of 
>> them ... Otherwise, I do not see any other options than hacking the code 
>> (but, again, others may have smarter ideas). SB
>>     
>
> Timo - what about the no_sym flag ? I thought that that would allow
> to force pwscf not to try to symmetrize the points (e.g. if
> one wanted to use a Baldereschi point, such as 1/4 1/4 1/4) - since
> it does not attempt to symmetrize anything, it might not use
> time reversal (rambling hypothesis - I'm sure SdG or PG know better,
> as always the case :-) ).
>
> As a side note - once k and -k are treated separately, even
> if the Hamiltonian is invariant for time reversal,
> in principle it's not psi(-k)=psi(k)* , but you might
> find more generally that psi(-k) = exp (i alpha) psi(k)* ,
> where alpha is random.
>
>
> 			nicola
>
>
> ---------------------------------------------------------------------
> Prof Nicola Marzari   Department of Materials Science and Engineering
> 13-5066   MIT   77 Massachusetts Avenue   Cambridge MA 02139-4307 USA
> tel 617.4522758 fax 2586534 marzari at mit.edu http://quasiamore.mit.edu
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-- 
______________________________________________________________________

Dr. TIMO THONHAUSER
Department of Materials Science and Engineering         (617) 253 6026
MIT, 13-4084, 77 Massachusetts Avenue               thonhauser at mit.edu
Cambridge, MA 02139-4307, USA                    www.mit.edu/~thonhaus
______________________________________________________________________