  From first principles, the author gathers a few general rules that need to be
abided by in the calculation of the internal partition functions (IPFs) of
individual molecules. These rules are violated in many schemes in the
literature where occupational probabilities are used including those using the
Planck-Larkin partition function (PLPF) within the chemical picture.
Considering these rules is useful from conceptual and practical points of view.
A solution is introduced to assure fulfilling the above mentioned rules when
using occupational probabilities. Sample calculations are performed showing
quantitative significance of inaccuracies caused by dishonoring such rules.
