NWChem erf partitioning

[susilehtola]

NWChem implements an error function based partitioning described in J. Chem. Phys. 152, 184102 (2020) on page 8

$$ w_A({\bf r}) = \prod_{B \neq A} \frac 1 2 [ 1 - \text{erf } \mu_{AB}'] $$

where

$$ \mu_{AB}' = \frac 1 \alpha \frac {\mu_{AB}} { [1 - \mu_{AB}^2]^{n}} $$

and

$$ \mu_{AB} = \frac {\mathbf{r}_A - \mathbf{r}_B} {|\mathbf{r}_A - \mathbf{r}_B|} $$