  In this work, the calculation of complexity on atomic systems is considered.
In order to unveil the increasing of this statistical magnitude with the atomic
number due to the relativistic effects, recently reported in [A. Borgoo, F. De
Proft, P. Geerlings, K.D. Sen, Chem. Phys. Lett., 444 (2007) 186], a new
analytical density to describe neutral atoms is proposed. This density is
inspired in the Tietz potential model. The parameters of this density are
determined from the normalization condition and from a variational calculation
of the energy, which is a functional of the density. The density is
non-singular at the origin and its specific form is selected so as to fit the
results coming from non-relativistic Hartree-Fock calculations. The main
ingredients of the energy functional are the non-relativistic kinetic energy,
the nuclear-electron attraction energy and the classical term of the electron
repulsion. The relativistic correction to the kinetic energy and the Weizsacker
term are also taken into account. The Dirac and the correlation terms are shown
to be less important than the other terms and they have been discarded in this
study. When the statistical measure of complexity is calculated in position
space with the analytical density derived from this model, the increasing trend
of this magnitude as the atomic number increases is also found.
