We present a simplified formalism for predicting infrared optical constants from first-principles calculations by considering resonant interaction between infrared photons and the crystal lattice. Addressing limitations of the widely used four-parameter semi-quantum Lorentz model, the proposed approach bridges the gap between the harmonic three-parameter model and the rigorous many-body Green’s-function-based approaches. By incorporating essential anharmonic effects including four-phonon scattering and phonon renormalisation, the model provides an efficient and accurate alternative while maintaining low computational cost. The frequency-dependent refractive indices of MgO and rutile TiO${}_2$ are computed and compared with experimental data, demonstrating good quantitative agreement. The framework offers a practical approach for predicting optical properties of materials across a wide range of materials.