We describe a general numerical method for the calculation of pseudorotation constants at an accuracy suited for thermodynamic applications. The pseudorotation is treated using classical mechanics along a pseudorotational pathway, which is spanned by molecular structures obtained from conventional ab initio geometry optimization. We present numerical results for systems with vertical pseudorotation arising from ring puckering (cyclopentane, cyclopentasilane) and for systems with in-plane pseudorotation arising from Jahn-Teller distortion of planar rings (benzene cation, cyclopentadienyl radical, cyclopropenyl radical).