The traceless molecular quadrupole moment, as defined by Buckingham [43], is calculated by using the keyword .QUADRU, and it can be requested from an input like:
**DALTON INPUT .RUN PROPERTIES **WAVE FUNCTIONS .HF **PROPERTIES .QUADRU **END OF DALTON INPUT
Note that both the electronic and nuclear contributions are always printed in the coordinate system chosen, that is, the tensors are not transformed to the principal axis system nor to the principal inertia system, as is often done in the literature.
The quadrupole moment is evaluated as an expectation value, and is thus fast to evaluate. This is noteworthy, because experimentally determined quadrupole moments obtained through microwave Zeeman experiments (see e.g. [44,45]) are derived quantities and prone to errors, whereas the calculation of rotational g factors and magnetizability anisotropies (see Chapter 10)--obtainable from such experiments--are difficult to calculate accurately [46]. An input requesting a large number of the properties obtainable from microwave Zeeman experiments is (where we also include nuclear quadrupole coupling constants):
**DALTON INPUT .RUN PROPERTIES **WAVE FUNCTIONS .HF **PROPERTIES .MAGNET .MOLGFA .QUADRU .NQCC **END OF DALTON INPUT
Note that the program prints the final molecular rotational g tensors in the principal inertia system, whereas this is not the case for the magnetizabilities and molecular quadrupole moment.