Hybrid calculation of the partition functions of prolate top molecules:Using HOCl as the test case.

•We present an accelerated technique for calculating the molecular partition function.•The technique combines a limited sum-over-states calculation with a correction term.•The accuracy of this hybrid technique is validated by application to HOCl.•Convergence is much faster than with a pure sum-over-states (SOS) calculation.•The technique is applicable to prolate top molecules, including quasilinear ones.