  The nature of the stereochemically active lone pair has long been in debate.
Here, by application of our recently developed orbital selective external
potential (OSEP) method, we have studied the microscopic mechanism of
stereochemically active lone pairs in various compounds. The OSEP method allows
us to shift the energy level of specific atomic orbital, therefore is helpful
to identify unambiguously the role of this orbital to the chemical and physical
properties of the system we are interested in. Our numerical results, with
compelling proofs, demonstrate that the on-site mixing of cation valence s
orbital with the nominally empty p orbitals of the same subshell is crucial to
the formation of lone pair, whereas the anion p orbital has only small effect.
Our detailed investigation of Sn and Pb monochalcogenides show that structures
of these systems have significant effects on lone pairs. In return, the
formation of lone pair, which can be controlled by our OSEP method, could
result in structural instabilities of Sn and Pb monochalcogenides.
