A rigid, nonlinear molecule has rotational energy levels determined by three rotational constants, conventionally written A , B , and C , which can often be determined by rotational spectroscopy. In terms of these constants, the rotational partition function can be written in the high temperature limit as ζ ζ rot ≈ ≈ π π σ σ ( k B T ) 3 A B C with σ σ again known as the rotational symmetry number which in general equals the number ways a molecule can be rotated to overlap itself in an indistinguishable way, i.e. that at most interchanges identical atoms. Like in the case of the diatomic treated explicitly above, this factor corrects for the fact that only a fraction of the nuclear spin functions can be used for any given molecular level to construct wavefunctions that overall obey the required exchange symmetries. Another convenient expression for the rotational partition function for symmetric and asymmetric tops is provided by Gordy and Cook: ζ ζ rot ≈ ≈ 5.34 × × 10 6 σ σ T 3 A B C where the prefactor comes from ( π π k B ) 3 h 3 = 5.34 × × 10 6 when A, B, and C are expressed in units of MHz.