On the accurate prediction of the optical absorption energy of F-centers in MgO from explicitly correlated ab initio cluster model calculations

A systematic study of the different computational requirements that affect the accuracy of the ab initio prediction of excitation energies of F and F centers on cluster models of MgO is reported. It is found that rather limi ted basis sets are enough to predict excitation energies of the F and F+ ce nters that are near to each other as experimentally observed. However, the absolute value of the excitation energy is in error by similar to1 eV or si milar to 20%. Increasing the basis set reduces the calculated excitation en ergy for the allowed transition, reaching a value of 5.44 eV for the F cent er, only 9% in error with respect to experiment. Improving the basis set do es not result in a better value of the excitation energy of the charged Fcenter. Attempts to improve the calculated result by geometry optimization of the region near the oxygen vacancy, enlarging the cluster model, improvi ng the primitive Gaussian set, or enlarging the auxiliary basis set centere d on the vacancy failed to further reduce the error. It is concluded that m uch larger basis sets are required to predict excitation energies of electr ons trapped at oxygen vacancies in ionic oxides with accuracy of or better than 0.4 eV. (C) 2001 American Institute of Physics.