# Wigner's $9$-$j$ symbol - four electrons orbital angular momentum coupling

In constructing the total angular momentum operator, that is the sum of 4 independent angular momentum operators:

$$J=J_1+J_2+J_3+J_4$$ one has the following set of commuting operators and eigenvectors in case of the uncoupled configuration:

$${\textbf{J}_1^2,J_{1z},\textbf{J}_2^2,J_{2z},\textbf{J}_3^2,J_{3z},\textbf{J}_4^2,J_{4z}},$$

$$|j_1m_1\rangle|j_2m_2\rangle|j_3m_3\rangle|j_4m_4\rangle.$$

Now in the coupled representation, if one couple pairwise there are different choices. Most commons I have seen in books are:

$$|(j_1j_2)J_{12}(j_3j_4)J_{34};J\rangle \ and\ |(j_1j_3)J_{13}(j_2j_4)J_{24};J\rangle$$

Now the Wigner 9j symbols are within a constant the coefficients that allow us to go from one basis to another, thus

$$\langle (j_1j_2)J_{12}(j_3j_4)J_{34};J|{(j_1j_3)J_{13}(j_2j_4)J_{24};J}\rangle \propto \Bigg\{ \begin{matrix} j_1 & j_2 & J_{12}\\ j_3 & j_4 & J_{34}\\ J_{13} & J_{24} & J\\ \end{matrix} \Bigg\}.$$

I am more interested in

extend coupling to n angular momenta by successive coupling of an extra angular momentum to the former n - 1 system [1]

In this case we have [2,3]

$$\langle [(j_1j_2)J_{12},j_3]J_{123},j_4;J|{[(j_4j_2)J_{42},j_3]J_{423},j_1;J}\rangle \propto \Bigg\{ \begin{matrix} j_2 & J_{12} & j_{1}\\ J_{42} & j_3 & J_{423}\\ j_{4} & J_{123} & J\\ \end{matrix} \Bigg\}.$$

and here my question and my doubt:

Is it possible to relate the following coupling scheme through a Wigner symbol ?

1. $$\langle [(j_1j_2)J_{12},j_3]J_{123},j_4;J|{[(j_2j_3)J_{23},j_4]J_{234},j_1;J}\rangle$$
2. $$\langle [(j_1j_2)J_{12},j_3]J_{123},j_4;J|{[(j_1j_2)J_{12},j_4]J_{124},j_3;J}\rangle$$

If yes, how can I build the Wigner 9j symbol (i.e. the positions of the $$j$$)? Is there any symbolic calculator or table where I can look for? It will really help me since I would like to extend the same also to the Wigner 12j and so on.

References

[1] Professor Dr. Kris L. G. Heyde - The Nuclear Shell Model - Study Edition (1994). pp 26

[2] Edmonds - Angular momentum in quantum mechanics-Princeton, N.J., Princeton University Press (1957). pp 104

[3] Albert Messiah - Quantum Mechanics. 2 - John Wiley and Sons, Inc. (1961). pp 1067

[4] A. P. Yutsis - Mathematical Apparatus of the Theory of Angular Momentum.

• I think that since this can be seen as a recoupling of three of the four angular momentum (where two terms are of the type $j$ and one of the type $J_{i,k}$), the wigner's coefficient between these two coupling schemes reduces to Wigner's 6-j symbol.
– 081N
Jun 29 '20 at 13:39
• what do you mean by "relate the scheme"? Are you asking if 1. and 2. can be expressed in terms of standard 9-j symbols or are you asking if one can define this overlap, compute from the get-go, and figure out if the resulting symbols as 9-j-like properties? Jun 29 '20 at 16:38
• I'm asking if 1. and 2. (each one separately) can be expressed in terms of 9-j symbols. Looking to Yutsis' graphs I think 1. and 2. can be expressed as 6-j symbols as they can be seen as recoupling of three angular momentum (I'll post a more detailed description). As you said you have some contrains in building the 9-j coefficient and at first glance it seems not possible with the coupling chosen.
– 081N
Jun 29 '20 at 18:00
• Yeah I don' have Jutsis with me but it's an extremely good reference, and I would not be surprised if you could express your overlap as a sum of 6j's. Jun 29 '20 at 18:11

In your first coupling scheme, you are forced by the scheme on the left to have $$j_1,j_2$$ and $$J_{12}$$ on the same row, and then $$J_{12}, j_3$$ and $$J_{123}$$ in the same column, and then $$J_{123}, j_4$$ and $$J$$ on the same row, i.e something like \begin{align} \left\{\begin{array}{ccc} j_1&j_2&J_{12}\\ &&j_3\\ J&j_4&J_{123}\\ \end{array}\right\} \tag{1} \end{align} But then for your scheme on the right you have $$j_2,j_3\to J_{23}$$ so $$j_2,j_3$$ and $$J_{23}$$ must be either on the same row or column. However, you can see this is not possible since $$j_3$$ must already be in the same column as $$J_{12}$$ and $$J_{123}$$, and cannot be in the same row as $$j_2$$ by your left coupling.
The same logic applies to your second coupling scheme, where from the left you'd have again (1) but this time you need to fit $$J_{12}$$ on the same line or column as $$j_4$$ when it's already in a given full line or column.
There are calculators but I don't think they will be useful since I don't think what you want is equivalent to a $$9j$$. You probably have to build a custom function using the same idea of writing the standard 9j as a sum of 6j's. Moreover, in your second scheme, you can probably reorganize \begin{align} [J_{12}j_4]J_{124}j_3 \to [J_{12}j_3]J_{123}j_4 \end{align} using a $$6j$$ symbol and make mileage there but again you'd have to do this as a custom function.
Note that it should not come as a surprise to get a negative answer. Historically the 9j was introduced to deal with the undoing of $$jj$$ coupling back to an LS coupling to compute matrix elements of - say - the spin-orbit coupling or some tensor operator acting on angular momenta only. Hence by design it is not constructed for recursive coupling of the type you want.