This keyword specifies a Welch potential[69,70] of the form

E

=

+ A_{ij}exp(- r_{ij}^{eff}/ρ) - -

-3 + + ,

where

= + - , r_{ij}^{eff} = .

Welch parameters in atomic units should be
entered after the Welch keyword in the order A_{++}, A_{–}, A_{+-}, ρ, Q_{+},
Q_{-}, α_{+}, α_{-}.
The ions are then specified using atom types PL and MI for plus and minus, respectively,
and can be in any order. There need not be equal numbers of positive and negative ions.