The authors report on spin polarised energy band calculations for the cubic Laves phase compound NpAl₂. The calculations were performed within the local spin density formalism, with a correction that takes into account the different intra-atomic Coulomb interactions for states with different azimuthal quantum numbers. The spin-orbit interaction was included in the band Hamiltonian and thus the parameter-free calculations took into account Hund's first, second and third rules. A description of the formalism is presented and it is found that only when treating all of Hund's rules simultaneously do they obtain good agreement between the theoretical and experimental magnetic moments. A prediction of the magnetic form factor is also presented. It is found that the cohesive properties are strongly affected when introducing the orbital polarisation, and only when doing this do the theoretical and experimental volumes compare well.