Nonempirical Study of Nuclear Dynamics in Nonrigid LiReO4 and K2SO4 Molecules

The previously developed nonempirical model for M_kXY_n (k 1) nonrigid molecules describing the motion of k M nuclei relative to a quasirigid XY_n fragment is used to study the dynamics of nuclei in LiReO₄ and K₂SO₄ molecules. The parameters of the kinetic and potential parts of the model Hamiltonian and dipole moment functions are determined from results of ab initio calculations of the molecules and their fragments by the Hartree—Fock and CISD + Q configurational interaction methods. The dynamic problem is solved by the variational method. Energy levels, transition frequencies, transition dipole moments, and mean geometrical parameters of the molecules are calculated. It is shown that the lowest energy levels of LiReO₄ and K₂SO₄ molecules can be described with high accuracy in a harmonic approximation using a quasirigid, single–minimum model, whereas for the energy levels located near and above the barriers of intramolecular rearrangements, these approximations are completely unsuitable.