The electron-nucleus contact term

The attempt to correct the non-relativistic Schrodinger equation in an approximate way for relativistic effects leads to the appearance of an one-electron operator, known as electron-nucleus Darwin term [109], 

We only briefly mention that a similar modification, i.e., a change from a Dirac delta distribution to an extended distribution, would be required for the spin-dependent electron-nucleus contact term, known as Fermi contact term, if the usual point-like nuclear magnetization distribution (the pointlike nuclear magnetic dipole approximation) is replaced by an extended nuclear magnetization distribution. 

---