Cu2+ impurities in tetragonal layered perovskites display a compressed geometry (R-eq > R-ax) along the crystal c-axis. Searching the microscopic origin of this fact DFT calculations have been performed on clusters simulating K2MgF4:Cu2+. Results for a 37 atoms cluster indicate that the difference, 6,c, between the energies for elongated and compressed minima along the c-axis is around 0.2 eV while R-eq -R-ax < 0.10 Angstrom. This result is shown to come mainly from the tetragonal electrostatic potential due to the rest of ions on CuF64-which provides a gap, Delta, between similar tox(2)-y(2) and similar to3z(2)-r(2) orbitals when R-eq = R-ax. Although delta(ec) < Delta this gap is shown to play a key role for understanding the equilibrium geometry in tetragonal perovskites. (C) 2004 Elsevier B.V. All rights reserved.