Solvent effects in four‐component relativistic electronic structure theory based on the reference interaction‐site model

A combined method of the Dirac–Hartree–Fock (DHF) method and the reference interaction‐site model (RISM) theory is reported; this is the initial implementation of the coupling of the four‐component relativistic electronic structure theory and an integral equation theory of molecular liquids. In the method, the DHF and RISM equations are solved self‐consistently, and therefore the electronic structure of the solute, including relativistic effects, and the solvation structure are determined simultaneously. The formulation is constructed based on the variational principle with respect to the Helmholtz energy, and analytic free energy gradients are also derived using the variational property. The method is applied to the iodine ion (I−), methyl iodide (CH3I), and hydrogen chalcogenide (H2X, where X = O–Po) in aqueous solutions, and the electronic structures of the solutes, as well as the solvation free energies and their component analysis, solvent distributions, and solute–solvent interactions, are discussed. A theory that includes both relativistic and solvent effects at a high level is crucial to elucidate the structure, properties, and reactions of molecules containing heavy elements in solution. We present a method that combines the Dirac–Hartree–Fock method, a four‐component relativistic electronic structure theory, with the reference interaction‐site model, an integral equation theory of molecular liquids. It thereby reveals the electronic structure, including relativistic effects, and solvation structure of molecules in solution.