We have investigated the correlation factors exp(-deltar_(12)),r_(12) exp(-deltar_(12)),erfc(deltar_(12)),and r_(12) erfc(deltar_(12)) in place of the linear-r_(12) term for use in explicitly correlated electronic-structure methods.The accuracy obtained with all of these correlation factors is significantly greater than that obtained with the plain correlation factor r_(12).Polarization functions that are more diffuse than those of standard basis sets give even better results.The correlation factor exp(-deltar_(12)) is very close to the optimum correlation factor for helium and outperforms the others.
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