Here, Eqs. 8-11 apply to GEO , but one can define analogous equations for GEO. As we shall see, the third term on the r.h.s. of Eq. 11 accounts for most of GEO , and this enables its intuitive interpretation in terms of the errors in individual geometric parameters. Instead of using internal coordinates to partition GEO , one can also use the errors in GEO normal modes (the eigenvectors of H in e.g., Cartesian coordinates). The advantage of the latter analysis is that GEO s becomes exactly equal to GEO , but the errors in these coordinates are less chemically intuitive than errors in e.g., bond lengths. This is why we proceed with the GEO analysis in internal coordinates.