Correlation consistent basis sets vibrational frequency convergence

For the vibrational frequencies and rotational constants, density functional theory calculations with the relatively modest 6-31IG basis set generally suffice. However, for accurate estimates of the lowest frequency modes it is occasionally necessary to include diffuse functions in the vibrational analyses and so it is perhaps better to employ the 6-311++G basis. Truhlar and coworkers have recently noted the related importance of diffuse functions in estimating relative conformational energies with density functional theory [47]. For density functional calculations there appears to be little rationale for employing the more expensive correlation-consistent basis sets, since the results are essentially converged with the 6-311-H-G basis set and since the limited accuracy of the density functional methods does not warrant extrapolation to the infinite basis set limit. 

---