- #1

- 397

- 0

## Main Question or Discussion Point

The Wikipedia article on the Helium Atom goes through the Thomas-Fermi approximation and shows that when you use a screening parameter Z you can optimize the energy of the atom according to the following formula:

[tex]\langle H \rangle = [-2Z^2 + \frac{27}{4}Z]E_1 [/tex]

([tex]E_1[/tex] is elsewhere defined as the Rydberg unit -13.6 eV)

The minimum value for energy occurs for a choice of Z=1.69, and you can plug in the numbers (I did) to confirm that this gives a value of -77.45 eV for the helium atom.

QUESTION: The article then concludes by saying that using this value "we obtain the most accurate result yet:"

[tex]\frac{1}{2} \Bigg(\frac{3}{2}\Bigg)^6 = -77.5 eV [/tex]

What is this formula??? Of course it is a mistake because it is missing [tex]E_1[/tex]; but that's not the problem. We can correct the mistake and just multiply 729/128 by 13.6 eV to get the quoted result. But how does this come from the original calculation? Where is the value Z=1.69 which came from the optimization?

[tex]\langle H \rangle = [-2Z^2 + \frac{27}{4}Z]E_1 [/tex]

([tex]E_1[/tex] is elsewhere defined as the Rydberg unit -13.6 eV)

The minimum value for energy occurs for a choice of Z=1.69, and you can plug in the numbers (I did) to confirm that this gives a value of -77.45 eV for the helium atom.

QUESTION: The article then concludes by saying that using this value "we obtain the most accurate result yet:"

[tex]\frac{1}{2} \Bigg(\frac{3}{2}\Bigg)^6 = -77.5 eV [/tex]

What is this formula??? Of course it is a mistake because it is missing [tex]E_1[/tex]; but that's not the problem. We can correct the mistake and just multiply 729/128 by 13.6 eV to get the quoted result. But how does this come from the original calculation? Where is the value Z=1.69 which came from the optimization?

Last edited: