PROBING THE ACCURACY OF PSEUDOPOTENTIALS FOR TRANSITION-METALS IN QUANTUM MONTE-CARLO CALCULATIONS

The accuracy of scalar-relativistic energy-consistent small-core pseud opotentials has been tested in quantum Monte Carlo calculations for th e first-row transition metals Sc, V, Ti, and Cr. We have calculated at omic ionization and excitation energies using variational and pure dif fusion quantum Monte Carlo methods as well as coupled cluster theory w ith nearly complete basis sets up to g functions. On the basis of the results we are able to estimate the magnitude of various errors relate d to the use of pseudopotentials in quantum Monte Carlo calculations f or transition metals. From a comparison of coupled cluster valence-onl y and all-electron results, respectively, of the experimental data we estimate the errors due to the semilocal pseudopotential to be at most 0.1 eV. The average error of pure diffusion quantum Monte Carlo resul ts with respect to experimental data amounts to 0.2 eV and arises main ly from errors due to the semilocal pseudopotential, its localization and the fixed-node approximation used in the quantum Monte Carlo calcu lations. The errors appear to be closely related to the occupation of the 3d shell. (C) 1997 American Institute of Physics.