Diffuse Function

In die Pople family of basis sets, the presence of diffuse functions is indicated by a + in die basis set name. Thus, 6-31- -G(d) indicates that heavy atoms have been augmented with an additional one s and one set of p functions having small exponents. A second plus indicates the presence of diffuse s functions on H, e.g., 6-311- -- -G(3df,2pd). For the Pople basis sets, die exponents for the diffuse functions were variationally optimized on the anionic one-heavy-atom hydrides, e.g., BH2 , and are die same for 3-21G, 6-3IG, and 6-3IIG. In the general case, a rough rule of thumb is diat diffuse functions should have an exponent about a factor of four smaller than the smallest valence exponent. Diffuse sp sets have also been defined for use in conjunction widi die MIDI and MIDIY basis sets, generating MIDIX+ and MIDIY-I-, respectively (Lynch and Truhlar 2004) the former basis set appears pardcularly efficient for the computation of accurate electron affinities. 

In die Dunning family of cc-pVnZ basis sets, diffuse functions on all atoms are indicated by prefixing with aug . Moreover, one set of diffuse functions is added for each angular momentum already present. Thus, aug-cc-pVTZ has diffuse f, d, p, and s functions on heavy atoms and diffuse d, p, and s functions on H and He. An identical prescription for diffuse functions has been used by Jensen (2002) in connection with the pc-n basis sets. 

Particularly for die calculation of acidities and electron affinities, diffuse functions are absolutely required. For instance, the acidity of HF (not Hartree-Fock in this case, but hydrogen fluoride) increases by 44 kcal/mol when the 6-3H-G(d) basis set is used instead of unaugmented 6-31G(d). 

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For all calculations, the choice of AO basis set must be made carefully, keeping in mind the scaling of the two-electron integral evaluation step and the scaling of the two-electron integral transfonuation step. Of course, basis fiinctions that describe the essence of the states to be studied are essential (e.g. Rydberg or anion states require diffuse functions and strained rings require polarization fiinctions). 

Basis sets can be extended indefinitely. The highest MOs in anions and weakly bound lone pairs, for instance, are very diffuse maybe more so than the most diffuse basis functions in a spht valence basis set. In this case, extra diffuse functions must be added to give a diffuse augmented basis set. An early example of such a basis set is 6-31+G [26]. Basis sets may also be split more than once and have many sets of polarization functions. 

Basis sets can be constructed using an optimisation procedure in which the coefficients and the exponents are varied to give the lowest atomic energies. Some complications can arise when this approach is applied to larger basis sets. For example, in an atomic calculation the diffuse functions can move towards the nucleus, especially if the core region is described... 

Corrections to the MP4/6-311G(d,b) Energy. Higher-level basis functions, if they are prudently chosen, should be better than lower-level functions. Thus the energy of, for example, a diffuse function, [MP2/6-311 - - G(d,p)] should be lower (more negative) than the same function in which diffuse electron density is not taken into account [MP2/6-31 lG(d,p)], provided that the levels of elecUon... 

One or two plus signs can also be added, such as 6—31+G or 6—31++G. A single plus sign indicates that diffuse functions have been added to atoms other than hydrogen. The second plus sign indicates that diffuse functions are... 

An extension of this last notation is aug—cc—pVDZ. The aug denotes that this is an augmented basis (diffuse functions are included). The cc denotes that this is a correlation-consistent basis, meaning that the functions were optimized for best performance with correlated calculations. The p denotes... 

Bauschlicker ANO Available for Sc through Cu (20.vl5/il0r/6/4 ). cc—pVnZ [n = D, T, Q, 5,6) Correlation-consistent basis sets that always include polarization functions. Atoms FI through Ar are available. The 6Z set goes up to Ne only. The various sets describe FI with from i2s p) to [5sAp id2f g) primitives. The Ar atoms is described by from [As pld) to ils6pAd2>f2g h) primitives. One to four diffuse functions are denoted by... 

Diffuse functions are those functions with small Gaussian exponents, thus describing the wave function far from the nucleus. It is common to add additional diffuse functions to a basis. The most frequent reason for doing this is to describe orbitals with a large spatial extent, such as the HOMO of an anion or Rydberg orbitals. Adding diffuse functions can also result in a greater tendency to develop basis set superposition error (BSSE), as described later in this chapter. 

A different scheme must be used for determining polarization functions and very diffuse functions (Rydberg functions). It is reasonable to use functions from another basis set for the same element. Another option is to use functions that will depict the electron density distribution at the desired distance from the nucleus as described above. 

Density functional theory calculations have shown promise in recent studies. Gradient-corrected or hybrid functionals must be used. Usually, it is necessary to employ a moderately large basis set with polarization and diffuse functions along with these functionals. 

The diffusion stage, characterized by H(t), will be discussed in detail with respect to the instantaneous wetting condition. However, in the presence of a time dependent wetting function (p(t), Eq. 2.1, we see from Eig. 2 that diffusion will have progressed to different extents in different areas of the interface. If the intrinsic diffusion function, H(t) as given by Eq. 1.1 does not change its nature with time due to the other stages, then the net diffusion, H"(t) can be expressed as the convolution product... 

After scaling, the predicted frequencies are generally within the expected range for carbonyl stretch (-1750 cm ). The table below reproduces our values, published theoretical values using the 6-31+G(d) basis set (this basis set includes diffuse functions), and the experimental values, arranged in order of ascending experimental frequency ... 

This table provides an introduction to the basis set effects we U discuss in the next chapter. Adding diffuse functions lowers the frequency by about 20-30 cm. However, both sets of numbers are in reasonable agreement with the observed values, with the better theoretical values producing quite good agreement. However, even using the smaUer basis set, we can successfully identify the carbonyl stretch. 

The general strategy embodied by this table is to select the most accurate calculation that is computationally practical for a given size system. Note that for the lower-cost methods, you will also need to add diffuse functions and/or additional polarization functions on the hydrogen atoms as appropriate for the systems you are studying. 

Split Valence Basis Sets Polarized Basis Sets Diffuse Functions Pseudopotentials... 

Diffuse functions are large-size versions of s- and p-type functions (as opposed to the standard valence-size functions). They allow orbitals to occupy a larger region of spgce. Basis sets with diffuse functions are important for systems where electrons are relatively far from the nucleus molecules with lone pairs, anions and other systems with significant negative charge, systems in their excited states, systems with low ionization potentials, descriptions of absolute acidities, and so on. 

The 6-31+G(d) basis set is the 6-31G(d) basis set with diffuse functions added to heavy atoms. The double plus version, 6-31++G(d), adds diffuse functions to the hydrogen atoms as well. Diffuse functions on hydrogen atoms seldom make a significant difference in accuracy. 

Diffuse functions have very little effect on the optimized structure of methanol but do significantly affect the bond angles in negatively charged methoxide anion. We can conclude that they are required to produce an accurate structure for the anion by comparing the two calculated geometries to that predicted by Hartree-Fock theory at a very large basis set (which should eliminate basis set effects). 

Even larger basis sets are now practical for many systems. Such basis sets add multiple polarization functions per atom to the triple zeta basis set. For example, the 6-31G(2d) basis set adds two d functions per heavy atom instead of just one, while the 6-311++G(3df,3pd) basis set contains three sets of valence region functions, diffuse functions on both heavy atoms and hydrogens, and multiple polarization functions 3 d functions and 1 f function on heavy atoms and 3 p functions and 1 d function on hydrogen atoms. Such basis sets are useful for describing the interactions between... 

The values in red are within O.OlA of the experimental value. Using the 6-31G basis set, including diffuse functions on the hydrogen atom, improves the result over that obtained with diffuse functions only on the fluorine atom, although the best result with this basis set is obtained with no diffuse functions at all. 

All of the geometries predicted with the 6-31IG basis set are quite accurate. Adding two sets of diffuse functions yields a more accurate structure. However, adding additional polarization functions does not significantly affect the results. 6-311++G(d,p) thus appears to achieve the basis set limit for this model chemistry. ... 

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