First principles study of the electronic structure and bonding of Mn2

We have examined the electronic structure and bonding of the Mn-2 molecule through multireference variational calculations coupled with augmented quadruple correlation consistent basis sets. The Mn atom has a S-6(4s(2)3d(5)) ground state with its first excited state, D-6(4s(1)3d(6)), located 2.145 eV higher. For all six molecular states (1)Sigma(+)(g), (3)Sigma(+)(u), (5)Sigma(+)(g), (7)Sigma(+)(u), (9)Sigma(+)(g), and (11)Sigma(+)(u)(1) correlating to Mn(S-6)+Mn(S-6), and for six undecets, i.e., (11)Pi(u), (11)Sigma(+)(g), (11)Delta(g), (11)Delta(u), (11)Sigma(+)(u)(2), and (11)Pi(g) with end fragments Mn(S-6)+Mn(D-6), complete potential energy curves have been constructed for the first time. We prove that the bonding in Mn-2 dimer is of van der Waals type. The interaction of two Mn S-6 atoms is hardly influenced by the total spin, as a result the six Sigma states, singlet ((1)Sigma(+)(g)) to undecet ((11)Sigma(+)(u)(1)), are in essence degenerate packed within an energy interval of about 70 cm(-1). Their ordering follows the spin multiplicity, the ground state being a singlet, X (1)Sigma(+)(g), with binding energy D-e (D-0)approximate to 600 (550)cm(-1) at r(e)approximate to 3.60 A. The six undecet states related to the Mn(S-6)+Mn(D-6) manifold, are chemically bound with binding energies ranging from 3 ((11)Pi(g)) to 25 ((11)Pi(u))kcal/mol and bond distances about 1 A shorter than the states of the lower manifold, Mn(S-6)+Mn(S-6). The lowest of the undecets is of Pi(u) symmetry located 30 kcal/mol above the X (1)Sigma(+)(g) state. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.2993750]