# I Hartree fock derivation problem

1. Apr 3, 2016

### georg gill

Can anyone show the algebra that leads to the rewriting that I have underlined in the attachment. It is a derivation

It is taken from the start of a derivation of the roothans method of hartree fock from 1972 with the authors Snow and Bills.

2. Apr 3, 2016

### Staff: Mentor

Plugging in the Hamiltonian, you can group its terms into three parts:
One involving only $\phi_1$-related terms, one only involving $\phi_2$-related terms, and one with $r_{12}$. In the first two, you can take the other field out of the integral, where it then cancels the corresponding expression in the denominator.
Then use the symmetry (1<->2) to combine both simplified fractions. The third fraction from the original expansion just stays as it is.

3. Apr 3, 2016

### ChrisVer

write down your Hamiltonian... some part of it is going to see only the $\phi_1$'s and some other part will only see the $\phi_2$'s (that's the reason of the 2* integral)....
So you can break the integral $\int \int f(x_1,x_2) dx_1 dx_2 = \int \int f(x_1) f(x_2) dx_1 dx_2 = \int f(x_1) dx_1 \int f(x_2) dx_2$...that's the first integral in your orange... The second integral in your orange comes from the Hamiltonian's part that mixes $r_{1},r_2 \text{ into } r_{12}$ and so your double integral remains : $\int \int f(x_1,x_2) dx_1 dx_2$ which you cannot seperate.

I'm pretty sure this should go to quantum physics XD

4. Apr 3, 2016

### georg gill

Thanks I did get it from your explanation!