Notes. Comparison between the direct sum values and the classical approximation. Nuclear spin degeneracy is considered as 1 (for more details, see Appendix A). ^(a)Rotational partition function computed with the classical approximation for slightly asymmetric tops (Herzberg 1991). For HCOOCH₃ we computed this rotational partition function using the rotational parameters A, B and C of Tudorie et al. (2012) in the Principal Axis System. ^(b)Rotational partition function computed as a direct sum with Eq. (7). The rotational energies (A-symmetry v_t = 0 levels) are computed up to J = 79 with the Hamiltonian parameters of Carvajal et al. (2007). ^(c)Relative differences.