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The SSH model describes states of electrons in a polyacetylene chain, which is modeled as a lattice with two orbitals per site. Now, in many articles it is claimed that in the ground state, half of all the energy eigenstates (those with non-positive energies) will be filled due to the Pauli exclusion principle.

This means, the number of electrons is half the total number of sites.

But, does not every atom contribute a pi electron which can delocalize over the system? Then, the number of electrons will be equal to the number of oribitals, and the ground state will be a fully filled band.

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  • $\begingroup$ What articles claim this, for example? $\endgroup$ – Ruslan May 10 at 16:56
  • $\begingroup$ Aren’t there two electrons in those energy states? $\endgroup$ – Superfast Jellyfish May 10 at 17:03
  • $\begingroup$ Spins? Now I get it. $\endgroup$ – Archisman Panigrahi May 10 at 17:52
  • $\begingroup$ Generally we are interested in the model at half-filling, as then the Fermi energy lies in the gap and corresponds to an insulator. Since electron have two possible spin states, half filling corresponds to once electron per site, or two electrons per unit cell. $\endgroup$ – user111476 May 12 at 20:20
  • $\begingroup$ AFAIK there is no hindrance to consider the model at different levels of fillings. However, the chiral symmetry is only present at half-filling, hence the interest in this case. $\endgroup$ – PPR Aug 19 at 4:09

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