the potential energy curves of most stable diatomic molecules in their
ground states are well-known from spectroscopy and the curves for many
excited states are also known
for polyatomic molecules our knowledge is much less and it must be
realized that even for triatomic molecules the potential energy surface
is a function of the three variables that are needed to specify the
relative positions of the nuclei and hence a complete diagrammatic
representation of such surfaces is difficult
--
the term 'observable' in quantum mechanics goes further than referring
to that which we can actually see and encompasses any quantity that
can in principle be measured
there is an important distinction between a wavefunction whose form depends
on the choice of coordinates (variables) in which the system is described
and which is not an observable and, for example, a dipole moment which
has a value that should be independent of any technique for calculating it
or measuring it
--
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