# Implementing simple atom model using density functional theory (DFT)

I am trying to write computer code which will find the energy and density function for a atom with Z protons and N electrons. I am working in 1D for simplicity and would like to make the overall code as simple as possible (I am using Hartree units aswell). I am using also using the Hartree approximation. This is my overall algorithm:

1. Finds hydrogen like wave-functions (i.e. no electron-election interaction)
2. Works out density function via $n(z) = \sum_i \left | \phi_i(z) \right |^2$

3. Works out Hartree potential $V_H(r) = \int_0^\infty \frac{n(\vec{s})}{\left | \vec{r}-\vec{s} \right |} d\vec{s}$

4. Uses Kohn-Sham equation to find new individual wavefuncitons $\left (-\tfrac{1}{2}\bigtriangledown^2 + \frac{Z}{\left |r \right |} + V_H(\vec{r}) + V_{EX}(\vec{r}) \right )\phi_i = \varepsilon_i \phi_i$

5. Go back to step 2. and repeat

I am having a number of difficulties and would be very grateful for some help with any of my questions.

Firstly to compute the Hartree potential I need to turn it into a 1D problem, so we assume even spherical distribution of charge:

$V_H(r) = \int_0^\infty 4\pi s^2 \frac{ n(s)}{\left |r-s\right |} ds$

However this blows up when r = s! I have been looking into Gauss's law but had no luck.

Any help would be great.

• Cross-posted to Physics. – dmckee Mar 15 '12 at 20:31
• You might want to break this up into two questions: one about the singularity in the potential, and one about the exchange-correlation term. – Dan Mar 15 '12 at 20:52
• I agree, the singularity question can be treated both mathematically and computationally, so I'm going to keep it. Exchange-correlation should be a different question. – Aron Ahmadia Mar 16 '12 at 7:48
• Why are you forcing 1D physics? The Hartree term in 3D is non-singular! $V_H=4\pi/3\int n(r,\phi,\theta, ...) r \sin \theta dr d\theta d\phi$. What is your exchange term? – Deathbreath Mar 17 '12 at 1:52