Default value for the parameter anc_a of the species_full_anc
species, the parameter a of the analytic norm-conserving regularized Coulomb
potential of [Gygi, J. Chem. Theory Comput.19, 1300 (2023)]. This is
useful for specifying multiple atoms without specifying the species block.
It has the dimension of an inverse length, and the code uses the scaled radius
$a Z r$, so the regularization length is $1/(aZ)$ bohr and
depends on the atomic number. The default value of 4 has only been validated for
hydrogen; for a heavy atom it puts the regularization far below any usable grid
spacing, and a smaller value should be used.
It is only used by species_full_anc and is ignored for the other
all-electron types.