Theoretical Insights into the Structure, Bonding, and Nonlinear Optical Properties of Rb2Co(SeO3)2: A DFT and QTAIM Study

In this study, we report the hydrothermal synthesis and comprehensive quantum chemical characterization of the double selenite compound Rb2Co(SeO3)2. The structural, electronic, and optical properties were investigated using Density Functional Theory (DFT) in combination with Natural Bond Orbital (NBO) and Quantum Theory of Atoms in Molecules (QTAIM) analyses. The results confirm the coexistence of strong polar covalent Co–O bonds and weaker ionic Rb···O interactions, indicative of a mixed ionic–covalent bonding framework. Frontier molecular orbital (FMO) analysis revealed a moderate HOMO–LUMO energy gap (2.873 eV), suggesting balanced reactivity and stability, while the Molecular Electrostatic Potential (MEP) and natural charge distribution pointed to clearly defined nucleophilic and electrophilic regions. Calculated global reactivity descriptors further supported the compound’s potential for charge-transfer interactions. Importantly, the evaluation of nonlinear optical (NLO) properties, including dipole moment (μ), polarizability (α), and first-order hyperpolarizability (β), demonstrated a significant NLO response (βtot = 627.253 au), highlighting Rb2Co(SeO3)2 as a promising candidate for photonic and optoelectronic applications. Overall, this work contributes to the understanding of structure–property relationships in transition-metal selenites and illustrates the value of theoretical methods in predicting and tuning the functional properties of advanced inorganic materials.