Default value for the parameter gaussian_width. This is useful
for specifying multiple atoms without specifying the species block. The
default value is taken from the recommendation in
Phys. Rev. B55, 10289 (1997).
Note that this is a width in units of the grid spacing, not a length: the
Gaussian exponent is $\zeta = 1/(2\sigma^2\delta^2)$ with
$\delta$ the spacing. Reducing Spacing at fixed
$\sigma$ therefore shrinks the physical size of the nucleus, so a
spacing convergence study is not a convergence study of a single Hamiltonian.
It is only used by species_full_gaussian and is ignored for the other
all-electron types.