Electronic structure and spin coupling of the manganese dimer: The state of the art of ab initio approach

A thorough ab initio study of the Mn-2 dimer in its lowest electronic states that correlate to the ground Mn(S-6)+Mn(S-6) dissociation limit is reported. Performance of multireference methods is examined in calculations of the fully spin-polarized S=5((11)Sigma(+)(u)) state against the recent accurate single-reference coupled cluster CCSD(T) results [A. A. Buchachenko, Chem. Phys. Lett. 459, 73 (2008)]. The detailed comparison reveals a serious disagreement between the multireference configuration interaction (MRCI) and related nonperturbative results on the one hand and the complete active space perturbation theory (CASPT) calculations on the other. A striking difference found in the CASPT results of the second and third orders indicates poor perturbation expansion convergence. It is shown that a similar problem has affected most of the previous calculations performed using CASPT2 and similar perturbative approximations. The composition of the active space in the reference multiconfigurational self-consistent field calculations, the core correlation contribution, and basis set saturation effects are also analyzed. The lower spin states, S=0-4, are investigated using the MRCI method. The results indicate a similar dispersion binding for all the spin states within the manifold related to the closed 4s shells, which appears to screen and suppress the spin coupling between the half-filled 3d atomic shells. On this premise, the full set of model potentials is built by combining the accurate reference CCSD(T) interaction potential for S=5 and the MRCI spin-exchange energies for the S < 5 states. This approach leads to the value of 550 cm(-1) as a lower bound for the (1)Sigma(+)(g) ground-state dissociation energy. The spin-exchange energies themselves are found to comply with the simple Heisenberg model. The effective spin-coupling parameter J is estimated as -3.9 cm(-1), a value roughly 2.5 times smaller in magnitude than those measured in the inert gas cryogenic matrices. Compressing of the Mn-2 dimer in the matrix cage is suggested as the prime cause of this disagreement.