Coulomb singularity

The bare Coulomb operator

$V(\vert\mathbf{r}-\mathbf{r}'\vert)=\frac{1}{\vert\mathbf{r}-\mathbf{r}'\vert}$

in the unscreened HF exchange has a representation in the reciprocal space that is given by

$V(q)=\frac{4\pi}{q^2}$

It has a singularity at $q=\vert\mathbf{k}'-\mathbf{k}+\mathbf{G}\vert=0$, and to alleviate this issue and to improve the convergence of the exact exchange with respect to the supercell size (or the k-point mesh density) different methods have been proposed: the auxiliary function methods, probe-charge Ewald (HFALPHA), and Coulomb truncation methods (HFRCUT). These mostly involve modifying the Coulomb Kernel in a way that yields the same result as the unmodified kernel in the limit of large supercell sizes. These methods are described below.

Truncation methods

In this method the bare Coulomb operator $V(\vert\mathbf{r}-\mathbf{r}'\vert)$ is truncated by multiplying it by the step function $\theta(R_{\text{c}}-\left\vert\mathbf{r}-\mathbf{r}'\right\vert)$, and in the reciprocal this leads to

$V(q)=\frac{4\pi}{q^{2}}\left(1-\cos(q R_{\text{c}})\right)$

whose value at $q=0$ is finite and is given by $V(q=0)=2\pi R_{\text{c}}^{2}$. The screened Coulomb operators

$V(\vert\mathbf{r}-\mathbf{r}'\vert)=\frac{e^{-\lambda\left\vert\mathbf{r}-\mathbf{r}'\right\vert}}{\left\vert\mathbf{r}-\mathbf{r}'\right\vert}$

and

$V(\vert\mathbf{r}-\mathbf{r}'\vert)=\frac{\text{erfc}\left({-\lambda\left\vert\mathbf{r}-\mathbf{r}'\right\vert}\right)}{\left\vert\mathbf{r}-\mathbf{r}'\right\vert}$

have representations in the reciprocal space that are given by

$V(q)=\frac{4\pi}{q^{2}+\lambda^{2}}$

and

$V(q)=\frac{4\pi}{q^{2}}\left(1-e^{-q^{2}/\left(4\lambda^2\right)}\right)$

respectively. Thus, the screened potentials have no singularity at $q=0$. Nevertheless, it is still beneficial for accelerating the convergence with respect to the number of k-points to multiply these screened operators by $\theta(R_{\text{c}}-\left\vert\mathbf{r}-\mathbf{r}'\right\vert)$, which in the reciprocal space gives

$V(q)=\frac{4\pi}{q^{2}+\lambda^{2}} \left( 1-e^{-\lambda R_{\text{c}}}\left(\frac{\lambda}{q} \sin\left(qR_{\text{c}}\right) + \cos\left(qR_{\text{c}}\right)\right)\right)$

and

$V(q)=\frac{4\pi}{q^{2}} \left( 1-\cos(qR_{\text{c}})\text{erfc}\left(\lambda R_{\text{c}}\right) - e^{-q^{2}/\left(4\lambda^2\right)} \Re\left({\text{erf}\left(\lambda R_{\text{c}} + \text{i}\frac{q}{2\lambda}\right)}\right)\right)$

respectively, with the following values at $q=0$:

$V(q=0)=\frac{4\pi}{\lambda^{2}}\left(1-e^{-\lambda R_{\text{c}}}\left(\lambda R_{\text{c}} + 1\right)\right)$

and

$V(q=0)=2\pi\left(R_{\text{c}}^{2}\text{erfc}(\lambda R_{\text{c}}) - \frac{R_{\text{c}}e^{-\lambda^{2}R_{\text{c}}^{2}}}{\sqrt{\pi}\lambda} + \frac{\text{erf}(\lambda R_{\text{c}})}{2\lambda^{2}}\right)$