“…diagonalize the entire eight-dimensionalĤ 0 matrix instead of diagonalizing the two separate 2 × 2 submatrices of the block diagonal matrixĤ 0 =Ĥ 0 fs +Ĥ 0 Z When asked to determine the first order corrections to the energies for the intermediate field Zeeman effect for the n ¼ 2 degenerate subspace ofĤ 0 , some students correctly identified that one can initially choose either a basis consisting of states in the coupled representation or a basis consisting of states in the uncoupled representation and then diagonalizeĤ 0 ¼Ĥ 0 fs þĤ 0 Z in each degenerate subspace ofĤ 0 . For example, in a basis consisting of states in the coupled representation (jn; l; jm j i), the perturbation matrixĤ 0 ¼Ĥ 0 Z þĤ 0 fs corresponding to the n ¼ 2 subspace is given below [in which γ ¼ ðα=8Þ 2 13.6 eV, α ¼ e 2 =4πϵ 0 ℏc, β ¼ μ B B ext , and the basis states are chosen in the order j2; 0; 1 2 ; 1 2 i, j2; 0; 1 2 ; − 1 2 i, j2; 1; 3 2 ; 3 2 i, j2; 1; 3 2 ; − 3 2 i, j2; 1; 3 2 ; 1 2 i, j2; 1; 1 2 ; 1 2 i, j2; 1; 3 2 ; − 1 2 i, and j2; 1; 1 2 ; − 1 2 i]: However, when finding the corrections to the energy spectrum, some students attempted to diagonalize the entire 8 × 8Ĥ 0 matrix in the n ¼ 2 degenerate subspace ofĤ 0 . While this approach is correct, it is easier to diagonalize the 8 × 8Ĥ 0 matrix by diagonalizingĤ 0 only in the block diagonal subspaces with smaller dimensions than the initial 8 × 8Ĥ 0 matrix, i.e., the two separate 2 × 2 matrices…”