  Within quantum chemistry, the electron clouds that surround nuclei in atoms
and molecules are sometimes treated as clouds of probability and sometimes as
clouds of charge. These two roles, tracing back to Schr\"odinger and Born, are
in tension with one another but are not incompatible. Schr\"odinger's idea that
the nucleus of an atom is surrounded by a spread-out electron charge density is
supported by a variety of evidence from quantum chemistry, including two
methods that are used to determine atomic and molecular structure: the
Hartree-Fock method and density functional theory. Taking this evidence as a
clue to the foundations of quantum physics, Schr\"odinger's electron charge
density can be incorporated into many different interpretations of quantum
mechanics (and extensions of such interpretations to quantum field theory).
