Investigations on the spin Hamiltonian parameters and the local structures for the orthorhombic [CuX4(H2O)2]2− centers in NH4X (X = Cl, Br)

► EPR parameters and local structures are analyzed for Cu2+ centers in NH4X (X = Cl, Br). ► The related molecular orbital coefficients are quantitatively determined from the cluster approach. ► Ligand contributions are included from the cluster approach due to strong covalency. ► Ligand octahedra experience local bond length variations of the Jahn-Teller nature.