For example, R = I (the unit matrix) means that the laboratory frame and the moleculefixed frame coincide. The rotation matrix R can be parametrized with three Euler angles; see Section 2.2 below. The two frames are illustrated in Figure . In a particular electronic state i and orientation R with respect to the static magnetic field, we write E(i, R) for the total electronic energy of the molecule. The probability (density) to find the system in a given microstate in thermodynamic equilibrium is given by the Boltzmann factor