AbstractWithin the jellium model we write the vacancy formation energy Ef as the sum of several terms: Ef = ΔT + ΔExc + ΔEc + Er, where ΔT is the change in kinetic energy of the electrons, ΔExc is the change in exchange and correlation energy, ΔEc is the change in electrostatic energy, and Er is the contribution because of lattice relaxation. Each term in this equation, except Er, is calculated using the density functional formalism for Li, Na, K, Rb, and Cs, respectively. To obtain Er we used the corresponding experimental values of Ef and subtracted from each of them the corresponding calculated values of the other terms. The resulting values for Er are all positive and relatively large, varying from about 0.1 to 0.23 eV.