AB-INITIO CALCULATIONS OF RELATIVISTIC AND ELECTRON CORRELATION-EFFECTS IN POLYATOMICS USING THE UNIVERSAL GAUSSIAN-BASIS SET - XEF2

Ab initio accurate all-electron relativistic molecular orbital Dirac-F ock self-consistent field calculations are reported for the linear sym metric XeF2 molecule at various internuclear distances with our recent ly developed relativistic universal Gaussian basis set. The nonrelativ istic limit Hartree-Fock calculations were also performed for XeF2 at various internuclear distances. The relativistic correction to the ele ctronic energy of XeF2 was calculated as approximately -215 hartrees ( -5850 eV) by using the Dirac-Fock method. The dominant magnetic part o f the Breit interaction correction to the nonrelativistic interelectro n Coulomb repulsion was included in our calculations by both the Dirac -Fock-Breit self-consistent field and perturbation methods. The calcul ated Breit correction is approximately 6.5 hartrees (177 eV) for XeF2. The relativistic Dirac-Fock as well as the nonrelativistic HF wave fu nctions predict XeF2 to be unbound, due to neglect of electron correla tion effects. These effects were incorporated for XeF2 by using variou s ab initio post Hartree-Fock methods. The calculated dissociation ene rgy obtained using the MP2(full) method with our extensive basis set o f 313 primitive Gaussians that included d and f polarization functions on Xe and F is 2.77 eV, whereas the experimental dissociation energy is 2.78 eV. The calculated correlation energy is approximately -2 hart rees (-54 eV) at the predicted internuclear distance of 1.986 angstrom , which is in excellent agreement with the experimental Xe-F distance of 1.979 angstrom in XeF2. In summary, electron correlation effects mu st be included in accurate ab initio calculations since it has been sh own here that their inclusion is crucial for obtaining theoretical dis sociation energy (D(e)) close to experimental value for XeF2. Furtherm ore, relativistic effects have been shown to make an extremely signifi cant contribution to the total energy and orbital binding energies of XeF2. (C) 1995 John Wiley & Sons, Inc.