10.5: Pseudopotentials

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A drawback to using plane waves is that electron wavefunctions don’t actually look much like plane waves, so the basis set is very different from the wavefunctions, and very many Fourier components are required. One way around this is to use a ‘pseudopotential’ which attempts to describe the potential due to the nucleus and tightly bound shells of ‘core’ electrons which do not take part in bonding. In silicon for example the pseudopotential describes the nucleus and the \(1s2s2p\) electrons.

The pseudopotential can be deduced from properties of the perfect atom: Consider:

\[V (r) \Phi ({\bf r}) = E + \frac{\hbar^2 \nabla^2}{2m} \Phi ({\bf r}) \nonumber\]

Where we know atomic properties \(E\) and \(\Phi (r)\), but not \(V (r)\), the potential seen by the outer electrons. We can invert the Schrödingerproblem, solving for \(V (r)\) to give the exact \(\Phi (r)\) outside some core radius \(r > r_c\), but smoothing it out for \(r < r_c\).

In most applications involving chemical binding, the wavefunction only changes in the region outside \(r_c\). So although the pseudowavefunction is not the correct Kohn-Sham eigenfunction, changes in its energy due to interaction with other electrons and ions are the same as the change in the Kohn-Sham eigenfunction.

Choosing \(r_c\) and inverting the Schrödinger equation is non-unique, but in general:

Pseudopotentials depend on the \(l\) quantum number, because they must include the fact that, e.g. \(3s\) must be radially orthogonal to \(1s\) and \(2s\), while \(3d\) are automatically so because of the angular dependence. This is called non-locality.

The core charge produced by the pseudo wavefunctions must be the same as that produced by the atomic wavefunctions. This ensures that the pseudo atom produces the same scattering properties as the ionic core.

Pseudo-electron eigenvalues must be the same as the valence eigenvalues obtained from the atomic wavefunctions.

Pseudo wavefunctions must be continuous at the core radius as well as its first and second derivative and also be non-oscillatory.

If you find it surprising that this works - it is! However tens of thousands of calculations give energies correct to within a few percent, so the approach seems to accord well with reality.