The discovery of superconductivity in multilayer nickelates under pressure has intensified interest in understanding the magnetic and electronic properties of Ruddlesden-Popper nickelates. Using density functional theory with Hubbard corrections, we investigate the magnetic ground state, electronic structure evolution under pressure, and Sr-doping effects in ${\mathrm{La}}_2{\mathrm{NiO}}_4$. We find that at ambient pressure, tetragonal ${\mathrm{La}}_2{\mathrm{NiO}}_4$ exhibits $G$-type antiferromagnetic order with negligible interlayer magnetic coupling. Under hydrostatic pressure, the system undergoes a continuous insulator-metal transition at 50 GPa while maintaining robust magnetic order, contrasting sharply with the rapid magnetic suppression in ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$. Sr doping leads to a sequence of magnetic ground states in our calculations: from $G$ type to $A$ type, to striped antiferromagnetic orders, and, eventually, to ferromagnetic order, accompanied by metallization. Furthermore, ${\mathrm{LaSrNiO}}_4$ displays weak charge and orbital orders. These results reveal the unique pressure and doping effects of single-layer nickelates and provide insights into the magnetic mechanisms underlying nickelate superconductivity.

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• Electronic structure
• Pressure effects
• Ruddlesden-Popper compounds
• First-principles calculations