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The antiferromagnetism in an infinite-layer copper oxide CaCuO2 was analyzed by adding a well potential solely to the atomic potential of an O2− ion in constructing a basis set for a Bloch-type periodic linear combination of numerically calculated atomic orbitals. (The well is not added to the solid Hamiltonian.) In this method that solves a secular equation to find all electron states, the original Kohn−Sham local density approximation (LDA) was employed. (Magnetic properties calculated with generalized gradient approximation are more similar to those with the original Kohn−Sham LDA than Vosko−Wilk−Nusair results.) The Cu oxides contain the O2− ion, which is stable in a solid but emits an electron in a vacuum. This implies that the atomic wave equation of electrons for an isolated O2− in a vacuum lacks real solutions unless a well potential is added to the atomic potential. The well potential for the O2− basis set was calculated using the Coulomb potential generated by nuclei and electron clouds surrounding O2− in a solid. A relatively shallow well potential within an ion radius compared to the well depth usually used results in significant delocalization of electrons at the O2− site. This periodic molecular orbital method predicts insulating antiferromagnetic ordering in CaCuO2. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2012 [ABSTRACT FROM AUTHOR]