Used in the following papers:

Isolation, Structure and Electronic Properties of the Heterofullerene 
Salt K_6C_59N, K. Prassides et al. Science 271, 1833 (1996)

Molecular Structure and Chemical Bonding in K_3C_60 and K_6C_60,
W. Andreoni, P. Giannozzi, and M. Parrinello, Phys. Rev. B 51, 2087 (1995)