# Effect of time-reversal symmetry on SCF convergence

**URL:** <https://discourse.abinit.org/t/effect-of-time-reversal-symmetry-on-scf-convergence/3364>\
**Category:** Ground state\
**Created:** [June 27, 2023, 6:20pm UTC](https://discourse.abinit.org/t/effect-of-time-reversal-symmetry-on-scf-convergence/3364 "2023-06-27T18:20:35Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![iavas](https://discourse.abinit.org/letter_avatar_proxy/v4/letter/i/6de8d8/32.png) [@iavas](https://discourse.abinit.org/u/iavas)\
**Post date:** [June 27, 2023, 6:20pm UTC](https://discourse.abinit.org/t/effect-of-time-reversal-symmetry-on-scf-convergence/3364/1 "2023-06-27T18:20:35Z")

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Hello all,

I am trying to calculate the positron annihilation momentum distribution spectrum produced by a vacancy. The input file in the [tutorial](https://docs.abinit.org/tutorial/positron/#computing-the-electron-positron-momentum-distribution-doppler-spectrum-of-a-si-lattice) uses `istwfk *1`. After trying this, I found that this parameter severely affects the convergence of the SCF cycle. With all other parameters exactly the same, using the default value of `istwfk` can complete the GS calculation within no more than 100 SCF cycles, but using `istwfk *1` causes the total energy obtained from each SCF cycle to fluctuate dramatically, with no sign of convergence at all.

Is this normal? Do I need to use a higher `ecut` and `nkpt` to compensate for this?

And, what is the purpose of using `istwfk *1` to deactivate time-reversal symmetry in the above tutorial? If it is to get a uniform k-point grid in the momentum space, is that parameter still important if I only use the gamma point?

Thanks!

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**Author:** ![ebousquet](https://discourse.abinit.org/letter_avatar_proxy/v4/letter/e/67e7ee/32.png) [@ebousquet](https://discourse.abinit.org/u/ebousquet)\
**Post date:** [June 28, 2023, 10:53am UTC](https://discourse.abinit.org/t/effect-of-time-reversal-symmetry-on-scf-convergence/3364/2 "2023-06-28T10:53:46Z")

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Dear iavas,  
The divergence of the SCF can happen when you remove symmetries. This can be improved by reducing the diemix, diemixmag or increasing the nline flags.  
I do not not to which extend the istwfk=1 is important for the positron calculation but what about if you run the istwfk=1 calculation by reading the WFK or DEN file of the converged case with the default istwfk?  
Best wishes,  
Eric