 The triplets R"A"0 = ("R""A"10, "R""A"20, "R""A"30) satisfying these conditions, enter the theory as a given set of real constants.Following Biedenharn and Louck, we introduce an orthonormal body-fixed frame, the "Eckart frame",
If we were tied to the Eckart frame, which—following the molecule—rotates and translates in space, we would observe the molecule in its equilibrium geometry when we would draw the nuclei at the points,
Let the elements of R"A" be the coordinates with respect to the Eckart frame of the position vector of nucleus "A" (formula_1).Since we take the origin of the Eckart frame in the instantaneous center of mass, the following relation
holds.