We report computer based simulations of energetics, spectroscopy, and electron-phonon interaction of americium using a novel spectral density functional method. This approach gives rise to a new concept of a many-body electronic structure and reveals the unexpected mixed valence regime of Am $5f^6$ electrons which under pressure acquire the $5f^7$ valence state. This explains the unique properties of Am and addresses the fundamental issue of how the localization delocalization edge is approached from the localized side in a closed shell system.

• Received 23 July 2005

DOI:https://doi.org/10.1103/PhysRevLett.96.036404