The hydration of the uranyl dication: Incorporation of solvent effects in parallel density functional calculations with the program PARAGAUSS

The parallel density functional program PARAGAUSS has been extended by a tool for computing solvent effects based on the conductor-like screening model (COSMO). The molecular cavity in the solvent is constructed as a set of overlapping spheres according to the GEPOL algorithm. The cavity tessellation scheme and the resulting set of point charges on the cavity surface comply with the point group symmetry of the solute. Symmetry is exploited to reduce the computational effort of the solvent model. To allow an automatic geometry optimization including solvent effects, care has been taken to avoid discontinuities due to the discretization (weights of tesserae, number of spheres created by GEPOL). In this context, an alternative definition for the grid points representing the tesserae is introduced. In addition to the COSMO model, short-range solvent effects are taken into account via a force field. We apply the solvent module to all-electron scalar-relativistic density functional calculations on uranyl, UO₂²⁺, and its aquo complexes in aqueous solution. Solvent effects on the geometry are very small. Based on the model [UO₂(H₂O)₅]²⁺, the solvation energy of uranyl is estimated to be about -400 kcal/mol, in agreement with the range of experimental data. The major part of the solvation energy, about -250 kcal/mol, is due to a donor-acceptor interaction associated with a coordination shell of five water ligands. One can interpret this large solvation energy also as a compounded effect of an effective reduction of the uranyl moiety plus a solvent polarization. The energetic effect of the structure relaxation in the solution is only about 8 kcal/mol.