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GGA calculations also yield infinite susceptibility $^{9}$. As in Pd, or, for that matter, in Fe pnictides, fluctuations beyond the mean-field level should suppress magnetiza- tion and reduce the susceptibility to a large, but finite value. But, again as in Pd, strong spin fluctuations are bound to destroy any conventional s-wave superconduc- tivity, particularly given that with this relatively modest 

1 A. Ubaldini, E. Giannini, C. Senatore, D. van der Marel , arXiv:0911.5305 (unpublished). 2 

A. Pal, H. Kishan and V.P.S. Awana, arXiv:0912.0991 (un- published) 3 

I. R. Shein and A. L. Ivanovskii, Solid State Communica- tions, 150 , 640 (2010) 

carrier density electron-phonon coupling cannot be too strong. 

This leads us to the conclusion that the superconduc- tivity in the hole-doped BiOCu$_{1}$$_{−}$$_{y}$S must be unconven- tional, and probably magnetically mediated, although it is not clear yet what particular pairing symmetry can be generated in this material. 

One can question the validity of the virtual crystal ap- proximation, given that holes are introduced not through 

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substitution, but through Cu vacancies. To verify that, I have performed supercell calculations using four for- mula units with one vacancy, that is to say, Bi$_{4}$O$_{4}$Cu$_{3}$S$_{4}$ , corresponding to y = 0 . 25 . The calculations converged to a ferromagnetic state with M = 0 . 2 µ$_{B}$ per formula unit (0.8 µ$_{B}$ per supercell), again slightly below the half- metallic limit of 0.25 µ$_{B}$, thus confirming the validity of the virtual crystal calculations. Interestingly, the magne- tization was quite delocalized, with Cu ions carrying 0.1 – 0.13 µ$_{B}$, and the rest of the magnetic moment distributed among the sulfur ions and in the interstitial region. 

I thank Dirk van der Marel for stimulating discussions related to this work, and Peter Blaha for a technical con- sulation. I also acknowledge funding from the Office of Naval Research. 

4 H. Hiramatsu, H. Yanagi, T. Kamiya, K. Ueda, M. Hirano and H. Hosono Chem., Mater., 20 , 326 (2008) 5 

The full-potential LAPW method, as implemented in the WIEN2k packege, has been used for all calculations. Up to 840 inequivalent k-points have been utilized to achieved the self-consistency, with the energy convergency better then 

