Theoretical results for the electronic properties of the free base phthalocyanine monomeric and dimeric species are presented. The binding energy for the most stable dimer (a parallel-displaced structure) is 12 kcal/mol. The calculations of the electronic excitation spectrum rely on the analysis of structures generated by Born-Oppenheimer Molecular Dynamics at 500 +/- 4 K (monomer) and 700 +/- 4 K (dimer). The results for the absorption spectrum illustrate the importance of taking into account thermal broadening and dimerization. The Q(x)-Q(y) band splitting is similar to 40 nm in reasonable agreement with the experimental value of 64 nm at T = 783 K. (C) 2014 Elsevier B.V. All rights reserved.