A MRCI PS AND CASSCF STUDY OF THE GROUND-STATE MGO DISSOCIATION-ENERGY

Ab initio calculations at CASSCF and MRCI PS levels are used to determine the dissociation energy for the X 1-SIGMA+ state of MgO, which adiabatically dissociates to the ground state 1S(g) of magnesium and to the excited 1D(g) state of oxygen, as well as other spectroscopic parameters. Emphasis is placed upon the problem of properly selecting an adequate active space in CASSCF calculations and upon the improvements obtained in MRCI by selecting perturbatively the most important contributions to the total wavefunction and evaluating the remaining ones only by perturbational methods. Through a procedure based on stabilizing the computed dissociation energy, values of 3.87 eV (MRCI PS) and 4.20 eV (CASSCF) are obtained. These values compare with the experimental value of 3.76 +/- 0.13 eV.