Q. Geometry optimization of large molecules

 

A. The double-zeta polarization basis sets would be the smallest sets having acceptable accuracy. The correlated methods such as SAC-CI method have to calculate so-called one-body and two-body effective density matrices (EDM) to compute the energy gradient of a state. The EDMs are first calculated in the MO basis and transformed into the AO basis. This back-transformation is one of the time-consuming steps in the gradient calculations, and the transformation can be quickly done when the AO dimension is smaller. One may chose the valence double-zeta sets instead of the double-zera sets, because the AO dimension is smaller and the accuracy of the geometry is not so different.