Rare-earth (RE) doped optical materials are distinguished by their unique optical and electronic properties. These characteristics are essential for various applications, including laser stabilization, and phosphors. However, the sensitivity of these materials to environmental factors and intrinsic defects necessitates a deep understanding. Grasping how these factors influence the refractive index, absorption coefficient, and emission spectra is vital.This thesis delves into the effects of external loads on substitutional defects in laser host materials. We employ both first principles and additional theoretical methods for our investigations. While we cover materials from both the Orthosilicates and Orthovanadate categories, we place special emphasis on Y$_2$SiO$_5$ (YSO). This is due to its rising significance in applications like laser stabilization and quantum memory.Our discussion is split into two primary sections: ground-state and excited-state properties. In the ground-state section, we introduce a framework to comprehend variations in an optical cavity’s optical path. This is especially relevant when the refractive index changes because of mechanical stress, affecting its resonance frequency. Through this lens, we explore the inherent properties of Eu-doped YSO. Our insights span its mechanical and thermodynamic properties, leading us to a multi-scale approach. Our understanding extends to the effects of temperature and pressure, resulting in the identification of two essential ratios: dn/dP and dn/dT. These are vital for RE-doped YSO laser applications.For the excited-state properties, our focus shifts to the transition energy of the RE dopant within the host material. Current methods for studying the excitation of RE ions in these materials often encounter issues such as extensive computational demands and dependence on empirical data. We underline that techniques like GW+BSE and AIMPs aren’t universally applicable to all RE-doped materials. Instead, we advocate for the use of constrained DFT (cDFT), combined with hybrid DFT and spin-orbit coupling (SOC). This method pinpoints the 4f ground state position with less dependence on empirical data, offering a more efficient alternative to the Dorenbos model. It promises the potential to unveil new phosphors and provides a comprehensive analysis of RE-doped laser materials.