and use the harmonic oscillator-rigid rotor approximation (HO-RR) for Q rot and Q vib . If the energy difference between the ground and first electronic states is much larger than k B T, where T is the temperature, this assumption is reasonable. For the electronic partition function, the energies of the excited electronic states are calculated as discussed in the following Sec. 2.2. The total partition function, Q TOT , is the product of the translation mode partition function, Q tr , and the internal mode partition function, Q int .