Neutral plutonium (Pu I) is an exceptionally complex atomic system owing to its eight valence electrons, strong valence-valence and valence-core correlations, and pronounced relativistic effects. The resulting high density of states and small energy separations among levels with the same total angular momentum $J$ lead to strong configuration mixing, making reliable predictions of level energies and Landé $g$-factors particularly challenging. Configuration-interaction many-body perturbation theory (CIMBPT) can, in principle, treat these effects, but in practice often requires very large configuration spaces when the conventional Dirac-Hartree-Fock (DHF) starting potential is constructed for a closed-shell core. In this work, we employ a relativistic configuration-interaction perturbation-theory (CIPT) approach based on a DHF starting potential that explicitly includes six $5f$ electrons. This choice provides a more suitable starting Hamiltonian for Pu I than the conventional $V^{N-M}$ approximation used in CIMBPT, which for Pu I corresponds to removing all eight valence electrons ($V^{N-8}$) in constructing the self-consistent potential. It improves single-configuration energies and enables a more efficient treatment of configuration mixing at substantially reduced computational cost. Benchmark comparisons with available experimental data demonstrate good overall agreement for both energies and Landé $g$-factors. The present results indicate that CIPT provides an efficient and practical framework for describing the low-energy electronic structure of complex actinides and can be extended to other heavy atoms relevant to precision spectroscopy and symmetry-violation studies.

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• Atomic & molecular processes in external fields
• Atomic & molecular structure
• Atomic spectra
• Electronic excitation & ionization
• Electronic structure of atoms & molecules
• Relativistic & quantum electrodynamic effects in atoms, molecules,& ions
• Zeeman effect