Density functional B3LYP calculations with the 6-31G* and mixed 6-31G* (Fe) + 6-31G (C, H, N, O) basis sets have been performed to study the electronic and geometric structure, thermodynamic stability, and vibrational frequencies of the heme molecule FeC34H32N4O4, its ion FeC34H32N4O4+, and products of their fragmentation with sequential elimination of two carboxymethyl groups in the electronic states with different spin multiplicities. The fragmentation energies were refined with the larger 6-311++G** basis set. Results of the calculations are compared with available experimental data. The trends in the aforementioned properties with a change of the multiplicity, molecular charge, and the number of external carboxymethyl ligands present have been analyzed. (c) 2005 Elsevier B.V. All rights reserved.