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## Main Question or Discussion Point

What is the maximum size scale (how many atoms) where the concept of wave function is still useful in chemistry esp. biochemistry? For example. Wave function modelling is easily done in Hydrogen Atom. The interaction Hamiltonian is mostly of the nature E = K + P (Total Energy = Kinetic + Potential Energy) in wave form. So when you add energy to the electron and it rises up an orbital, the kinetic energy component in the wave function is increased. But what happens in the molecules. When you add energy, you only affect the bulk properties or thermal. Does one use the concept of Wave function here? If not, then up to what size scale (how many atoms) is wave function concept still meaningful?

Related to this is, if Hilbert Space formalism is used. Up to what size scale (how many atoms or size of molecules) is Hilbert Space concept still meaningful or useful?

In other words, what advantage do you have in modelling molecules using wave function or Hilbert Space and up to what size is it still accurate and useful??

Related to this is, if Hilbert Space formalism is used. Up to what size scale (how many atoms or size of molecules) is Hilbert Space concept still meaningful or useful?

In other words, what advantage do you have in modelling molecules using wave function or Hilbert Space and up to what size is it still accurate and useful??