1 Relativistic Quantum Calculations to Understand the Nature of f-Orbitals and Chemical Bonding of Actinides with Organic Ligands Srimanta Pakhira 1 , Andy D. Zapata-Escobar 2 , Joaquin Barroso-Flores 3,4 , Gustavo A. Aucar 2* and Jose L. Mendoza-Cortes 5,6,7* 1 Department of Physics, Department of Metallurgy Engineering and Materials Science (MEMS), Centre for Advanced Electronics (CAE), Indian Institute of Technology Indore, Simrol, Khandwa Road, Indore, 453552, Madhya Pradesh (M.P.), India. 2 Instituto de Modelado e Innovación Tecnológica, CONICET-UNNE, and Departamento de Física, FCENA-UNNE, Avda. Libertad 5460, W3404AAS, Corrientes, Argentina. 3 Instituto de Química, Universidad Nacional Autónoma de México (UNAM), Circuito Exterior SN, Ciudad Universitaria, Coyoacán CP 04510 Ciudad de México, México 4 Current address: Centro Conjunto de Investigación en Química Sustentable UAEM – UNAM, Carretera Toluca-Atlacomulco Km 14.5, CP 50200, Toluca, Estado de México, México. 5 Department of Physics, Scientific Computing, Material Sciences and Engineering, High-Performance Material Institute, Condensed Matter - High Magnetic Field National Lab, Florida State University, Tallahassee FL, 32310, USA. 6 Department of Chemical and Biomedical Engineering, FAMU-FSU Joint College of Engineering, Tallahassee, FL 32310, USA. 7 Current address: Department of Chemical Engineering & Materials Science, Michigan State University, East Lansing, Michigan 48824, USA. *Email: gaa@unne.edu.ar (G.A.A.), jmendoza@msu.edu (J.L.M-C.) Abstract: The nuclear waste problem is one of the main interests of the rare earth and actinide elements chemistry. Actinides are at the frontier of current theoretical methods due to the need to consider relativistic effects and approximations to the Dirac equation. Here, we employ four- component relativistic quantum calculations and scalar approximations to understand the nature of f-orbitals in the chemical bonding of actinides to organic ligands. We studied the relativistic quantum structure of an isostructural family made of Plutonium (Pu), Americium (Am), Californium (Cf), and Berkelium (Bk) atoms with the redox-active model ligand; DOPO (2,4,6,8- tetra-tert-butyl-1-oxo-1H-phenoxazin-9-olate). Crystallographic structures were available to validate our calculations for all mentioned elements except for Cf. State-of-the-art relativistic calculations were performed at different levels of theory to investigate their electronic structure: