The effect of uncertainty on building blocks in molecules.

Probabilities to find a chosen number of electrons in flexible domains of space are calculated for highly correlated wave functions. Quantum mechanics can produce higher probabilities for chemically relevant arrangements of electrons in these regions. However, the probability to have a given arrangement, e.g., that corresponding to chemical formulas (bonds or atoms), is low although being often maximal. Like in valence bond theory, it is useful to consider alternative distributions of electrons. Exchanges of electrons should be considered not only between atoms but also between other types of regions, such as those attributed to lone pairs. It is useful to have definitions flexible enough to allow users to find the most relevant representations. We tentatively suggest a tool (the effective number of parties) to help one make the choice.