Denoting the homogeneous external magnetic field as ⃗ B 0 , the total magnetic field felt by a magnetic nucleus is given by ⃗ B = ⃗ B 0 + ⟨ ⃗ B ind ⟩ tot . The rotation matrix R describing the relation between the laboratory frame and the molecule-fixed frame can be parametrized with three Euler angles: 24 θ and ϕ, the polar and azimuthal angles of z with respect to Z, and χ, which describes the rotation of the molecule-fixed frame around its own z axis. This Euler angle convention is also called the "extrinsic ZYZ" convention, because the molecule-fixed frame can be obtained by starting with the laboratory frame, then performing a rotation of all basis vectors around Z by angle χ, then performing a rotation around Y by angle θ, and finally performing a rotation around Z by angle ϕ. The rotation can be expressed in terms of the orthogonal rotation matrix or "direction cosine matrix" 24