We study the electronic structure of the ground state of the manganese dimer using the state-averaged complete active space self-consistent field method, followed by second-order quasidegenerate perturbation theory. Overall potential energy curves are calculated for the 1Σg+, 11Σu+, and 11Πu states, which are candidates for the ground state. Of these states, the 1Σg+ state has the lowest energy and we therefore identify it as the ground state. We find values of 3.29 Å, 0.14 eV, and 53.46 cm−1 for the bond length, dissociation energy, and vibrational frequency, in good agreement with the observed values of 3.4 Å, 0.1 eV, and 68.1 cm−1 in rare-gas matrices. These values show that the manganese dimer is a van der Waals molecule with antiferromagnetic coupling.