Basis sets, diffuse dimer

A comprehensive comparison of various basis sets for the homodimers of HF and H2O offers hope that calculation of vibrational frequencies can be meaningful, even when restricted to the SCF level and with no account of anharmonicity. The frequencies are less demanding of basis set quality than are the intensities. Minimal basis sets are to be avoided in most cases, as are small split-valence sets such as 3-21G. In some cases, one can compute reasonable estimates of dimerization-induced frequency shifts with basis sets of 4-3IG type however, results with unpolarized basis sets can be deceptive. Polarization functions are strongly recommended for uniform quality of results, particularly if one is interested primarily in spectral changes induced by H-bond formation. Intensity calculations without polarization functions can be expected to yield only the crudest of estimates. Reasonable results can be achieved with only one set of such functions on each atom. In some cases, it may be useful to include diffuse + functions as well. 

REF17 Tsuzuki, S., Uchimaru, T., Mikami, M. and Tanabe K. (1996) Basis set effects on the calculated bonding energies of neutral benzene dimers importance of diffuse polarization functions, Chem. Phys. Lett., 252, 206-210... 

Accurate ah initio calculations for fhe benzene dimer require using bofh an extensive basis set with diffuse funcfions and a high level of dynamic correlation. Recently, several independent studies were devoted to theoretical investigation of the potential energy surface of fhe benzene dimer [33,34,38-40]. This work closely follows the analysis presented by Sherrill and co-workers [33,34]. They estimated... 

The force constants for the free dimer system were obtained from frequency calculations using the Gaussian 03 code [69]. All calculations were performed using the B 3LYP [40,41 ] hybrid functional in combination with a triple- 6-311++G basis set, including diffuse functions on all atoms. 

Alagona G, Ghio C (1990) The effect of diffuse functions on minimal basis set superposition errors for Id-bonded dimers. J Comput Chem 11 930-942... 

The RVS analysis when applied to hydrogen-bonded systems behaves in many ways similar to the KM analysis, especially in the ELE and EX terms. The variation can generally be seen in the POL and CT terms, which are generally overestimated in the KM analysis. The overestimation becomes more apparent with the addition of diffuse function to the basis set. Results presented in Table 15.2 reinforce the above-mentioned fact. A similar observation has also been seen in the case of water dimer and trimers by Chen and Gordon. "... 

On the other hand, earlier calculations by Frisch et al. [94] with somewhat larger basis sets had indicated that the relative amounts of zero-point vibrational energy in the cyclic and linear dimers can reverse upon small changes in basis set, e.g. addition of diffuse functions. Furthermore, the use of the harmonic approximation on a surface as flat as this one is dubious, so the question remains incompletely resolved. 

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