Basis sets recommendations

In general, it is agreed that the FCP approach is a good correction to the energy, but an overcorrection may be introduced when small basis sets are used. " Nevertheless, the size of the overcorrection is small, and it is easily fixed by increasing the size of the basis set. In fact, it has been shown that this overcorrection decreases rapidly with basis sets size, even faster than other errors also introduced by small basis sets. Selection of the basis sets that properly describe the properties of the system being studied are perhaps more important than the size of the overcorrection. This topic is discussed later in the section about basis sets recommendations. 

It is emphasized in Ref. 187 that very flexible basis sets are required to deal with the finite-basis 0-dependence of the complex eigenvalues in CCR methods, and in particular to converge the resonance widths. However, the term very flexible is used in comparison to standard basis sets for valence anions, and in fact good results are obtained for auto-ionizing resonances of He, H , and Be using the aug-cc-pVTZ-l-[3s3p] basis,which includes three even-tempered diffuse s and p shells. This is not much different from the basis sets recommended here for proper description of loosely-bound electrons in general gas-phase calculations. 

Th e con traction expon en ts and cocfTicien ts of th e d-type functions were optinii/ed using five d-primitives (the first set of d-type functions) for the STO-XG basis sets and six d-primitives (the second set of d-type functions ) for the split-valence basis sets. Thus, five d orbitals are recommended for the STO-XG basis sets and six d orhitals for the split-valence basis sets. 

In order to do so, you will need to perform Hartree-Fock NMR calculations using the 6-311+G(2d,p) basis set. Compute the NMR properties at geometries optimized with the B3LYP method and the 6-31G(d) basis set. This is a recommended model for reliable NMR predictions by Cheeseman and coworkers. Note that NMR calculations typically benefit from an accurate geometry and a large basis set. 

The following table summarizes the most commonly-used basis sets and provides some recommendations as to when each is appropriate ... 

We do not recommend using this basis set as a matter of course unless you have opulent CPU resources. 

G(d) Use the 6-31G(d) basis set which is a useful and often-recommended basis set). 

In terms of basis sets, there is compelling evidence that sets smaller than polarized triple-zeta quality significantly reduce the accuracy that can be obtained with modem hybrid functionals and cannot be recommended if quantitative energetic results are the prime target. 

The SCF-MI method provides interaction energy values which converge quite rapidly with increasing basis set size. This fact makes this approach particularly recommended for large interacting moieties where basis sets of double- or triple-zeta quality are typically used and where the use of very extended basis set, like Millet-Stone s, is precluded. The resulting BSSE effect on the interaction densities should in this case be much larger than that found for the water dimer. 

The exchange repulsion energy in EFP2 is derived as an expansion in the intermolecular overlap. When this overlap expansion is expressed in terms of frozen LMOs on each fragment, the expansion can reliably be truncated at the quadratic term [44], This term does require that each EFP carries a basis set, and the smallest recommended basis set is 6-31-1— -G(d,p) [45] for acceptable results. Since the basis set is used only to calculate overlap integrals, the computation is very fast and quite large basis sets are realistic. 

Approximate linear dependence of AO-based sets is always a numerical problem, especially in 3D extended systems. Slater functions are no exceptions. We studied and recommended the use of mixed Slater/plane-wave (AO-PW) basis sets [15]. It offers a good compromise of local accuracy (nuclear cusps can be correctly described), global flexibility (nodes in /ik) outside primitive unit cell can be correct) and reduced PW expansion lengths. It seems also beneficial for GW calculations that need low-lying excited bands (not available with AO bases), yet limited numbers of PWs. Computationally the AOs and PWs mix perfectly mixed AO-PW matrix elements are even easier to calculate than those involving AO-AO combinations. 

---