1：Experimental Design and Method Selection:

The study used first-principles calculations based on density functional theory with the Vienna ab initio simulation package (VASP). The Perdew-Burke-Ernzerhof scheme of generalized gradient approximation was employed for exchange-correlation. Convergence tests determined parameters such as cut-off energy and k-points.

2：Sample Selection and Data Sources:

The binary Heusler alloy MgCl3 with DO3-type structure was modeled. Various (001) surface terminations (ClCl-term, Mg*Mg*-term, ClMg-term, ClCl*-term, Mg*Mg-term) were created by cleaving or substituting atoms.

3：List of Experimental Equipment and Materials:

Computational software VASP was used; no physical equipment or materials were specified.

4：Experimental Procedures and Operational Workflow:

Atomic displacements and surface energies were calculated. Electronic, magnetic, and optical properties were analyzed through density of states, band structures, and dielectric functions. Hydrogen-termination effects were tested by adding H atoms to surfaces.

5：Data Analysis Methods:

Data were analyzed using VASP outputs, with spin polarization, atomic magnetic moments, and optical coefficients derived from calculated properties.