 Polarized functions: In the case of the molecules, the one atomic orbitals are shifted towards one side or other side (means polarized) due to mixing of other orbitals, which distorts the orbital shape. In other words, the s orbital can polarize in one direction if it's mixed with a p orbital and the p orbitals can polarize if mixed with d orbitals. These polarization effect can be implemented in the basis set with consideration of additional functions of higher angular momentum. For example, in the case of ethylene, p functions sufficiently describe the π-bonds, but the d functions allow the electron density to bend away from the π-bond towards the hydrogen. The common polarised basis sets are 6-31G(d) and 6-311G(d,p).