Spin density functional calculations employing the full potential linearized augmented plane wave method (FLAPW) are performed on the periodic structure of sodium electro sodalite. The density functional adopted (PW91) includes gradient corrections for exchange and correlation. A body-centered cubic lattice of bare Na-4(3+) clusters is found to be metallic and diamagnetic. The presence of the aluminosilicate framework makes it an antiferromagnetic material with a gap of about 0.1 eV between valence and conduction band. The antiferromagnetic state is more than 110-170 kJ mol(-1) more stable than a ferromagnetic state. The Heisenberg exchange integrals between nearest and next nearest neighboring Na-4(3+) clusters, J(nn) and J(nnn), are derived from an extended Huckel tight-binding approach. The parameters of this Hamiltonian were chosen such that the density functional band structure is reproduced. The Heisenberg exchange integrals between nearest and next nearest neighboring Na-4(3+) sites, J(nn) and J(nnn), are negative, i.e., both nn and nnn sites are coupled antiferromagnetically. In absolute terms J(nnn) is 0.6 meV. Estimates of J(nn) are between 6.4 and 9.5 meV, the most likely value being 8.1 +/-0.5 meV. Using these values for J(nn) and J(nnn) the molecular field approximation yields Weiss temperatures between -160 and -230 K. The most likely result, -200 +/- 10 K, fits well to the experimental value of about -200 K.