Basis set convergence in second-row compounds. The importance of core polarization functions

Using sequences of Dunningʹs correlation consistent basis sets, cc-pVnZ (n=3,4,5), convergence of molecular properties is much slower for second-row compounds than for their first-row counterparts, both at the Hartree–Fock and at correlated levels, due to core polarization effects. By adding a single high-exponent d function to the standard cc-pVnZ basis sets, convergence is greatly accelerated. After correcting for core correlation, computed D0, re, and ωe values for a number of diatomics generally agree with experiment to better than 0.02 eV, 0.001 Å, and 5 cm−1, respectively.