For ease of computation using the computational resources of the 1950s, Nilsson used a basis consisting of eigenstates of the spherical hamiltonian. The Nilsson quantum numbers are { N , ℓ ℓ , m ℓ ℓ , m s } . The difference between the spherical and deformed Hamiltonian is proportional to r 2 Y 20 δ δ , and this has matrix elements that are easy to calculate in this basis. They couple the different N shells. Eigenstates of the deformed Hamiltonian have good parity (corresponding to even or odd N) and Ω, the projection of the total angular momentum along the symmetry axis. In the absence of a cranking term (see below), time-reversal symmetry causes states with opposite signs of Ω to be degenerate, so that in the calculations only positive values of Ω need to be considered.