Some basis sets are said to be "correlation consistent". What does it mean in practice ?
Wikipedia has an answer here:
Edit: adding introductory text from Wikipedia:
Some of the most widely used basis sets are those developed by Dunning and coworkers, since they are designed to converge systematically to the complete-basis-set (CBS) limit using empirical extrapolation techniques. For first- and second-row atoms, the basis sets are cc-pVNZ where N=D,T,Q,5,6,... (D=double, T=triples, etc.). The 'cc-p', stands for 'correlation-consistent polarized' and the 'V' indicates they are valence-only basis sets. They include successively larger shells of polarization (correlating) functions (d, f, g, etc.). More recently these 'correlation-consistent polarized' basis sets have become widely used and are the current state of the art for correlated or post-Hartree-Fock calculations.
I recommend reading http://jcp.aip.org/resource/1/jcpsa6/v90/i2/p1007_s1 for an answer to this question.