[Wannier] Complex values in seedname.mmn from real VASP wavefunction

[Espen Sagvolden]

Dear all,

Seemingly paradoxical values arise in the seedname.mmn files from wannier_setup with VASP: They seem to be complex-valued (alternating between purely real and purely imaginary), even when the underlying VASP wavefunction is real (gamma-point computation). Since the seedname.mmn entries should only be orbital overlaps, this troubles me, but I'm new to Wannier90 and there may be something I don't know about the overlap integrals.

Here is an excerpt from the seedname.mmn file for a subspace of the four lowest orbitals from a gamma-point VASP run with real orbitals:

File generated by VASP: unknown system
           4           1           4
    1    1    0    0    1
    0.946270648002   -0.000000000000
   -0.000695821546    0.000000000000
   -0.000000000000   -0.000000278197
   -0.000000000000   -0.000000195965
   -0.000695821546    0.000000000000
   -0.944393397524   -0.000000000000
   -0.000000000000    0.000000000281
   -0.000000000000   -0.000000001135
   -0.000000000000   -0.000000278197
   -0.000000000000    0.000000000281
    0.962394262259    0.000000000000
    0.000000000265   -0.000000000000
   -0.000000000000   -0.000000195965
   -0.000000000000   -0.000000001135
    0.000000000265   -0.000000000000
    0.962394258975    0.000000000000
    1    1    1    1   -1
    0.810376149699    0.000000000000
   -0.000596328721    0.000000000000
    0.000000000000    0.498834776389
    0.000000000000    0.103890215607
   -0.000596328721    0.000000000000
   -0.811164204017    0.000000000000
   -0.000000000000    0.000205733055
   -0.000000000000    0.000042846592
    0.000000000000    0.498834776389
   -0.000000000000    0.000205733055
    0.648824351005   -0.000000000000
   -0.046098789574   -0.000000000000
    0.000000000000    0.103890215607
   -0.000000000000    0.000042846592
   -0.046098789574   -0.000000000000
    0.860569485467   -0.000000000000
    1    1    1    0   -1
    0.810376181990   -0.000000000000
   -0.000596328947   -0.000000000000
    0.000000000000    0.159445743764
    0.000000000000    0.483948506733
   -0.000596328947   -0.000000000000
   -0.811164196521   -0.000000000000
   -0.000000000000    0.000065759196
   -0.000000000000    0.000199593417
    0.000000000000    0.159445743764
   -0.000000000000    0.000065759196
    0.847555958915   -0.000000000000
   -0.068638884681   -0.000000000000
    0.000000000000    0.483948506733
   -0.000000000000    0.000199593417
   -0.068638884681   -0.000000000000
    0.661838058801   -0.000000000000
    1    1    0    1    1
    0.810376169686   -0.000000000000
   -0.000596328168    0.000000000000
    0.000000000000    0.339388794748
   -0.000000000000   -0.380058459416
   -0.000596328168    0.000000000000
   -0.811164201039    0.000000000000
   -0.000000000000    0.000139974419
   -0.000000000000   -0.000156748296
    0.000000000000    0.339388794748
   -0.000000000000    0.000139974419
    0.767710591576   -0.000000000000
    0.114737676141    0.000000000000
   -0.000000000000   -0.380058459416
   -0.000000000000   -0.000156748296
    0.114737676141    0.000000000000
    0.741683353736   -0.000000000000

Yet, when seedname.mmn is used in a subsequent wannierization run with gamma_only = T, the resulting seedname_u.mat file seems to be OK (i.e., no imaginary components).

written on 25Nov2020 at 10:27:40
           1           4           4

   0.0000000000  +0.0000000000  +0.0000000000
   0.5773502345  +0.0000000000
  -0.0002035071  +0.0000000000
  +0.4419431296  +0.0000000000
  +0.6865514806  +0.0000000000
  +0.0003524826  +0.0000000000
  +0.9999999379  +0.0000000000
  +0.0000000032  +0.0000000000
  -0.0000000003  +0.0000000000
  -0.5773502553  +0.0000000000
  +0.0002035033  +0.0000000000
  +0.8155425747  +0.0000000000
  -0.0394582080  +0.0000000000
  -0.5773502102  +0.0000000000
  +0.0002035074  +0.0000000000
  -0.3735994902  +0.0000000000
  +0.7260097205  +0.0000000000


Is this because everything straightens itself out when the MLWF algorithm is run,
or are imaginary components stripped away at some point because of the keyword gamma_only = T?

Best regards,

Espen
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