Why computed entropies of quasi-linear species are sometimes random?

Using the example of a gaussian 90 computation of the linear BBNB molecule in its singlet state, it is shown that the calculated entropy value can be considerably different from that deduced from seemingly equivalent computations performed on a quasi-linear species. This problem is interpreted in terms of the limiting behaviour of the conventional rotational partition function of a polyatomic molecule. The problems originate from a routine application of quantum-chemical programs to such linear cases. The results are shown to be important for linear systems optimized (owing to, for example, better convergence properties) as quasi-linear systems. The finding explains why some published computed entropies of essentially linear species cannot be reproduced.