New correlation factors for explicitly correlated electronic wave functions

We have investigated the correlation factors exp(-zeta r(12)), r(12) exp(-zeta r(12)), erfc(zeta r(12)), and r(12) erfc(zeta r(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(-zeta r(12)) is very close to the optimum correlation factor for helium and outperforms the others. (c) 2005 American Institute of Physics.