Applications of field theoretical methods to the calculation of infrared band shapes of molecules in strongly interacting solvents. I.

Starting from a finite temperature Green’s function formalism we have developed a diagrammatic method for calculating the complex dielectric constant of a system consisting of diatomic molecules that are strongly interacting with the solvent medium. Perturbative treatments suitable for weak interactions have been replaced by a variational method, and diagrams picked up are summed up to all orders in the solvent–solute interaction potential. A method is presented whereby all the vectorial integrations can be carried out exactly leaving a final quadrature which must be done numerically. It is pointed out that the internal states of the diatomic molecule are coupled to each other via a very complicated cooperative effect induced by the solvent.