There are \( \left ( \begin{array}{c} 4 \\ 2 \end{array} \right) = 6 \) two-particle states,
which we list with the total \( M \):
| Occupied | \( M \) |
| 1,2 | 0 |
| 1,3 | -1 |
| 1,4 | 0 |
| 2,3 | 0 |
| 2,4 | 1 |
| 3,4 | 0 |
There are 4 states with \( M= 0 \),
and 1 each with \( M = \pm 1 \).