Basis sets geometry dependence

The VCD of 7 was recalculated in 1988 with the MFP method using analytical methods as implemented in CADPAC rather than finite difference methods. The DO gauge and substantially large basis sets were employed. The VCD spectrum was also remeasured and rotatory strengths were obtained. The basis set, geometry, and force field dependence the VCD intensities ot this molecule were investigated in those studies. 

The choice of the proper basis set clearly depends on the systems under investigation and/or property of interest (e.g., diffuse functions needed, polarization functions on hydrogen) as well as the method used (different requirements for HF and DFT as compared with correlated calculations), cost-accuracy considerations, and to some extent the program used (different programs might have different performance with the various types of basis sets). Sufficiently accurate molecular geometries can be obtained with smaller basis sets [e.g., 6-31G(d,p) or 6-31+G(d,p)] but possibly not reliable energies. 

Basis set dependence is important. The results in Table 16.1 were obtained for HF-LCAO calculations on pyridine. In each case, the geometry was optimized As a general rule, ab initio HF-LCAO calculations with small basis sets tend to underestimate the dipole moment, whilst extended basis sets overestimate it A treatment of electron correlation usually brings better agreement with experiment. 

If the pertiubation is a change in geometry, the basis set is clearly perturbation dependent since the functions move along with the nuclei. Standard perturbation theory is therefore not suitable for calculating molecular gradients. 

Table 2. Relative energies E (kJ mol-1) of the butyl cation dependent on the geometry and quantum chemical method used (data from 32) calculations with basis set 1-3 simple ab initio CEPA ab initio with electron correlation)...

The dependence of the DFT results on the basis set used to expand the Kohn-Sham orbitals is illustrated in Table 4.3, which collects equilibrium geometry properties of water dimer obtained with the same exchange-correlation functional (B88/P86) but with different basis sets. 

Table 1.5 The basis-set dependence of the AE of the CO molecule (kJ/mol). The calculations have been carried out at the all-electron CCSD(T) level at the geometry optimized in the cc-pCVQZ basis [25],...

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