We investigate the electronic structure of americium (Am) across its four experimentally confirmed high-pressure phases, Am-I ($P{6}_3/mmc$), Am-II ($Fm\overline{3}m$), Am-III ($Fddd$), and Am-IV ($Pnma$), up to 100 GPa, using density functional theory combined with an embedded dynamical mean-field approach. Our calculated density of states reproduces the key spectral features observed in ultraviolet photoemission spectroscopy for the Am-I phase. The results reveal a progressive pressure-driven evolution from purely localized $5f$ states of Am-I and Am-II toward the onset of delocalization. This trend is evidenced by a systematic spectral weight transfer toward the Fermi level, enhanced $f\text{−}c$ hybridization, and increasingly pronounced valence state fluctuations characterized by the mixing of $5f^5, 5f^6$, and $5f^7$ electronic configurations. Crucially, despite these delocalization trends, the $5f$ partial density of states remains strictly depleted at the Fermi level in all phases. In our calculations Am-IV represents a distinct “localized but hybridized” state, in which the $5f$ electrons retain their localized character while beginning to participate in bonding. This interpretation reconciles conflicting experimental reports and establishes Am as a quintessential platform for understanding the localized to itinerant transition in actinides. This unified picture not only resolves long-standing controversies surrounding the nature of Am's high-pressure phases but also provides a robust microscopic framework for predicting the behavior of neighboring transuranium elements under extreme conditions.

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• Density of states
• Electronic structure
• High-pressure studies
• Actinides
• Dynamical mean field theory
• Electron-correlation calculations