Symmetry and two electron wave function

[hokhani]

TL;DR

Understanding the relation between system symmetry and wave-function symmetry

In the picture below we have two identical orbitals A and B and the system has left-right symmetry. I use the notation $|n_{A \uparrow}, n_{A \downarrow},n_{B \uparrow},n_{B \downarrow}>$ which for example $n_{A \uparrow}$ indicates the number of spin-up electrons in the orbital A. I would like to know is it possible to have an eigenstate as $|1,1,0,0>$ in this left-right symmetric system or, because of the symmetry of system, we must only have symmetric wave-functions as $\frac{|1,1,0,0>\pm|0,0,1,1>}{\sqrt(2)}$?
Any help is appreciated.

 

[DrClaude]

The only symmetry that states must follow is the one related to interchange of two identical particles.

(That said, if you need states to be eigenstates of some operators, like the Hamiltonian, then certain symmetries may need to be respected. But for a general state, there is no such requirement.)

 

[hokhani]

Thanks very much. In the second quantized form, the interchange of particles are included in the commutation of fermionic operators.
However, I would like to know whether the left-right symmetry of this system demands the eigenstates of the Hamiltonian be in the form $\frac{1}{\sqrt(2)}(|1,1,0,0>\pm|0,0,1,1>)$ or they can also have the form like $|1,1,0,0>$?

 

[DrClaude]

What is the Hamiltonian?

 

[hokhani]

I don't mean a specific Hamiltonian. I mean each left-right symmetric Hamiltonian for this system.

 

[hokhani]

Sorry if I didn't convey myself. To explain in a better way; for a left-right symmetric system, can we have the eigenstates which doesn't have this symmetry?

 

[gentzen]

Sorry if I didn't convey myself. To explain in a better way; for a left-right symmetric system, can we have the eigenstates which doesn't have this symmetry?

If you have a degenerated eigenvalue with a left-right symmetric and a left-right anti-symmetric eigenstate, then take a superposition of them to get an eigenstate which is not left-right symmetric.

 

[hokhani]

If you have a degenerated eigenvalue with a left-right symmetric and a left-right anti-symmetric eigenstate, then take a superposition of them to get an eigenstate which is not left-right symmetric.

Thanks, I got it. Like the parity in one-particle system, provided that there is no degeneracy, the eigenstates must have even or odd symmetry as $\frac{1}{\sqrt(2)}(|1,1,0,0>\pm|0,0,1,1>)$. Otherwise, there is no demand for symmetry of the eigenstates.