Option trace

trace will calculate the trace of density times overlap matrix:

$$N = \{\mathbf{DS}\}$$

If the orbitals are orthonormal, $N$ should yield the total number of electrons in your molecule. If this is not true, your MO-vector will most probably be erroneous. For example, the vector might belong to another geometry or basis set. As this is a very sensitive test for errors like these and the calculation requires almost no time, you should always switch on this option.