## The molecular field approximation

MFA

$$\hat{A}\hat{B}\approx \hat{A}\langle\hat{B}\rangle+\hat{B}\langle\hat{A}\rangle-\langle\hat{A}\rangle\langle\hat{B}\rangle$$

What this mean physically? What we neglect here?

If I calculate Neel temperature using this method $$T_N^{MFA}$$ and using RPA method $$T_N^{RPA}$$ is there some relation between those temperatures?

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 mean-field approximation method
 Perhaps a natural way to see the decomposition is if A and B are fluctuations around some mean, i.e. = = 0. Then you just have AB ~ 0, i.e. you are neglecting correlated fluctuations in A and B.