The Role of Binary and Many-center Molecular Interactions in Spin Crossover in the Solid State. Part III. Expressing Many-body Interactions

Summary. The approach of molecular potentials describing the shape of transition curves of spin crossover in the solid state developed earlier has been extended to many-body interactions characterized by the Axilrod-Teller potential. An improved procedure for the minimization of energy developed for this case is presented. Calculations for systems involving Lennard-Jones, electric dipole–dipole, and dispersive Axilrod-Teller triple interactions yield non-zero asymmetries of splittings in expanded/compressed systems alone. The excess energy is unaffected by the Axilrod-Teller potential. Triple interactions of the Axilrod-Teller type thus increase the sensitivity of a transition curve towards compression. Another approach presented employs the deviations of molecules from positions of mechanical equilibrium set up by the known binary potential. In the approximation of small perturbations these deviations are proportional to the gradients of many-center potentials. This allows one to parametrically define non-ideality parameters as functions of gradients of triple potentials of unknown types. Employing regularization bounds an adequate parameterization of experimental transition curve of spin crossover has been achieved in terms of parameters of Lennard-Jones potential and relative deviations of molecules from the position of mechanical equilibrium.