Local density pseudopotential calculations for molecules: O2 and Mo2O2S2(S2)2−2

We report a development of a self-consistent, local density pseudopotential method for calculation of the electronic properties of large molecules. Angular momentum dependent, first-principles ionic pseudopotentials are used to describe the valence-core interactions. Average correlation effects are included in the local density sense. The wave functions, the charge density and the exchange-correlation potential are expanded in atom-centered Gaussian orbitals of s, p, and d symmetry. These atomic basis sets account for nonspherical (non-muffin tin) effects and are easily transferable to different geometries and other environments. The total energy of the system is accurately evaluated. The method is tested on the O2 molecule and found to give the orbital energies, the equilibrium distance and the vibrational frequency in good agreement with previous all-electron calculations. As the first application to a large system, we have investigated the electronic structure of the Mo2O2S2(S2)2−2 anion. In this anion, the molybdenum atoms are fivefold coordinated: to one oxygen atom, two bridging sulfur atoms, and a molecular S2. From the energy level diagram, the oxygen atoms are found to form approximately triple bonds to the molybdenum atoms, the Mo–S and Mo–S2 bonds being successively weaker. The molybdenum atoms form a single Mo–Mo bond and are formally in the Mo(V) oxidation state. The highest occupied molecular orbitals are, however, the S2 π* orbitals, oriented perpendicular to the Mo-S2 planes and located almost 1 eV above the Mo–Mo bond energy. The lowest unoccupied molecular orbitals are the relatively closely spaced π* antibonding Mo–O, Mo–Mo, and Mo–S2 orbitals.