In the periodic table of the elements there are 106 entries: every atom is identified by the number of protons (atomic number) contained in the nucleus. When the atomic number is increased, the nuclear charge increases too, augmenting the attraction between the nucleus and the electrons and therefore these last ones improve their speed, which value becomes comparable with the light speed.

In this case the Schroedinger equation is inappropriate and the one of Dirac, which takes into account relativistic effects, must be used. This correction is necessary for about half of the elements of the periodic table, therefore molecules with heavy atoms must be described by this methodology.

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Bibliography:

• E. Clementi, L. Pisani, "Dirac-Fock Self-Consistent Field Calculations for Closed-Shell Molecules with Kinetic Balance", , in METECC-94, Methods and Techniques in Computational Chemistry", vol. A, E. Clementi ed., STEF, Cagliari (1993).