# The central equation

• A
I went through the derivation for the central equation

$$(\lambda_k - \epsilon)C_k + \Sigma_G U_G C_{k-G} = 0$$

and everywhere I look everybody just says this is a NxN matrix. I don't see how this is true. Isn't this just a one line equation with summation across all reciprocal lattice? I don't understand why at one value of k we get NxN matrix. Any help is greatly appreciated!

Yeah this is what I found but the central equation is
$$\frac{\hbar^2k^2}{2m} C_k + \sum_{G}^{} V_GC_{k-G}=EC_k$$
so shouldn't I just have
$$\frac{\hbar^2k^2}{2m} C_k + V_0C_{k-g} + V_0C_{k+g}=EC_k$$
for G=ng and $$V=V_0e^{igx} +V_0e^{-igx} ?$$
How do you get the other equations
$$\frac{\hbar^2(k-g)^2}{2m} C_{k-g} + V_0C_{k-2g} + V_0C_{k}=EC_{k-g}$$

Lord Jestocost
Gold Member
The "central equation" is a set of equations. For a fixed k in the first Brillouin zone, this set of equations for all reciprocal lattice vectors G couples those coefficients Ck , CkG , CkG' , CkG'' ,... whose wave vector differ from k by a reciprocal vector. Equation (107) in [1] which follows the presentation in the textbook “Solid State Physics” by Neil W. Ashcroft and N. David Mermin should illustrate the meaning of the "central equation". Have a look at [2], too.

[1] [PDF]Introduction to Solid State Physics
[2] [PDF]7.6 The Schrodinger equation of electron in a periodic potential

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Well I read the book by ashcroft and mermin and [2] before. Everybody just says that the C(k) couples to C(k+G). Are they referring to the coupling via V0? I really don't see where the other equations come from

I guess I am asking why is C(k) a vector? Isn't C(k) just the Fourier coefficient of the wavefunction?