Relativistic valence-bond method with superposition of configurations for calculations of the electronic structure of diatomic molecules with heavy atoms

A new relativistic valence-bond method including configuration interaction is suggested for calculations of the electronic structure of diatomic molec ules. The basis set of one-electron orbitals includes relativistic Hartree- Fock atom-type orbitals obtained by numerically solving the Hartree-Fock-Di rac equations for free atoms-ions or atoms in an external field. The basis set also includes virtual atomic states described by the Hartree-Fock-Sturm relativistic orbitals. The complete wave function is obtained as a linear combination of Slater determinants constructed from localized atomic-type o rbitals nonorthogonal on different centers. Alongside covalent configuratio ns, ionic charge-transfer configurations are included in the calculations. In the one-configuration variant, the method is equivalent to the relativis tic Heitler-London model. The suggested method is designed for calculating molecules with heavy atoms; it is an all-electron method, which does not re quire the use of pseudopotentials or effective core potentials for excludin g core states. The use of the numerical Hartree-Fock basis set is especiall y effective in calculations of such physical characteristics as densities o n the nucleus, superfine splitting parameters, X-ray transition energies, c hemical shifts, etc. The calculation data on the spectral characteristics o f the AgH and AgH+ molecules are given. The relativistic calculation result s are compared with the data of nonrelativistic calculations performed with the use of the same basis set and in the same configuration space by a sim ilar nonrelativistic method.