to incorporate Q. To the best of our knowledge, this is the first derivation of the PBC electric quadrupole. Notice the resemblance in form with Eq. 23, where the main difference is the use of a symmetric direct product rather than a cross product, along with a change of sign in each term. As was the case with the Eq. 23, we need to ensure that Q is Hermitian and we again achieve this via explicit symmetrization, with the second term in Eq. 33 canceling out due to being anti-Hermitian.