Used in the following papers:

Anomalous electronic behavior of Na_xC_60 (x<=10): Theory and Experiment,
W. Andreoni, P. Giannozzi, J. F. Armbruster, M. Knupfer and J. Fink
Europhys. Lett. 34, 699 (1996)

Absence of metal clusters and appearance of new electron states in Na_6C_60
W. Andreoni, P. Giannozzi, and M. Parrinello, Phys. Rev. Letters 72, 848 (1994)