This term has been computed in earlier analytic derivative studies of internally contracted MRPTs (CASPT2 and NEVPT2), and the origin of the term is similar to the AO derivative contribution: the so-called connection term or the Pulay force. In the context of ICCs, it may be considered that the term arises in order to keep the orthogonal ICCs orthogonalized after perturbative displacements of atoms. More explicitly, we can take the partial derivative of, for example, the third term in Eq. ( ) and obtain