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Basis set extension effects

As in the two-electron case, the superscript denotes a quantity calculated in terms of the truncated set of NOs. The basis set extension effect (BSE) is calculated at the MP2-level. [Pg.96]

The problem of secondary BSSE, or basis set extension effects, has not been solved as yet. Only very approximate means have been devised for dealing with it. Fortunately, like primary BSSE at the SCF level, it does appear possible to employ basis sets that reduce the secondary error to an acceptably small magnitude. [Pg.179]

N. S. Ostlund and D. L. Merrifield, Chem. Phys. Lett., 39, 612 (1976). Ghost Orbitals and the Basis Set Extension Effects. [Pg.215]

Although both of these definitions suffer from the obvious limitations imposed by the arbitrariness of the Hilbert space partitioning, equation (17) has been found to produce bond orders that are somewhat less sensitive to the basis set extension effects. In light of the arguments given in the previous sections, neither of these definitions is suitable for rigorous analysis of electronic wavefunctions. The same is true for the shared electron numbers produced by the formalisms developed by Roby and others. ... [Pg.897]

VB model, though successful for the interactions between monovalent atoms, breaks down when 71 bonds are considered. The aim of this chapter is to bring a quantitative answer to a question which can be so summarized What is the nature of the driving force which makes benzene more stable in a D6h geometry than in an alternated Dih geometry of Kekule type Exactly the same type of question applies to the allyl radical which will also be investigated and will allow the study of the effects of configuration interaction (Cl) and basis set extension. [Pg.30]

Let us first use Eq. (1) to estimate the resistance to distortion arising from n interactions in the high spin states. The results are displayed in Table 2, and show a remarkable constancy at the three levels of computations used in the case of allyl. Note that when Cl is performed in the n space, the n component A of the distortion energy is not any more given by the three first terms in Eq. (1), but one can still compute it as the difference between the total distortion energy and its a component, calculated from the last three terms of Eq. (1). It turns out that A in the quadruplet allyl is effectively smaller than our higher limit of 0.5 kcal/mol, and does not appear to be sensitive to Cl or basis-set extension. Similar results are also observed in benzene and cyclobutadiene, in which the n systems of the high spin states also prove to be rather insensitive to the distortion. [Pg.33]

As shown in ref. [22], the effect of basis set extension is clearly visible the height of the peaks increased by 0.18 e A-3 for the C3=C4 double bond (Figure 6d), by 0.25 e A-3 for the C-N bonds, and by 0.24 e A-3 for C=0 bonds. For the C=0 region, extended basis sets reduce the depopulation region close to the oxygen atom, increase the peak height, displace it by 0.16 A towards the oxygen atom, and reduce the lone pairs accumulation by 0.25 e A-3. [Pg.276]

The term efSE represents a basis set extension (BSE) effect calculated at the MP2-level. As the dimension of the truncated orbital space is increased, the magnitude of the term e SE is reduced. [Pg.96]

For extensive basis sets, an optimal description of the subsystems X and Y and the supersystem X... Y will be obtained. The basis set superposition error will then be very small. In recent work. Wells and Wilson did not use the function counterpoise correction in the usual fashion described above. They pointed out not only that the Boys-Bemardi procedure overcorrects for basis set superposition effects but also that it cannot be uniquely generalized for the calculation of a many-body interaction. Wells and Wilson argue that the function counterpoise correction should be used as a test for basis set superposition errors. [Pg.479]

Pluta and Sadlej have calculated the dipole moment and static a, P and y and y tensors of urea and thiourea using three high level basis sets of increasing flexibility. Excellent agreement is found with experimental determinations of the dipole moment and linear polarizability. Frequeney-dependent polarizabilities and hyperpolarizabilities are ealeulated in the TDHF approximation and the results are then scaled to allow for electron correlation and the effect of basis set extension. Estimates of the response flmctions for non-linear optical processes are obtained. The introduction of the sulfur atom is found to produce a large increase in the predicted efficiency for third order effects. [Pg.309]

Scaling of the second-order correlation energy has been used to estimate the effects of basis set extension. Consider a calculation performed using a basis set designated Sa- The relation between the modified second-order correlation component and the higher-order approximation to the total correlation energy then takes the form... [Pg.354]

Bar MR, Sauer J (1994) Ab initio calculations of the stracture and properties of disiloxane. The effect of electron correlation and basis-set extension. Chem Phys Lett 226 405-412... [Pg.524]

Notice in the first place the effect of basis set extension in the results afforded by DFT methods. The most important effect is produced by the extension of the valence and diffuse part of the basis set, not by the addition of polarization functions. Extension from 6-31G(d,p) to 6-311 -PG(d,p) modifies the results for different reactions by 2-3 kcal/mol. Further extension of the polarization space from (d,p) to (3df,2pd) produces a tenfold smaller effect. The maximum absolute deviation (MAD) and root mean square error (RMSE) with respect to the experimental results obtained using reported standard enthalpies of formation show for almost all methods the counterintuitive result that an improvement in the basis set actually worsens the agreement with experiment. Only in the case of the T-dependent BMK DFT method, developed specifically for thermochemical and kinetic studies, the increase in the basis sets results in better agreement with the experiment. [Pg.69]

Enthalpies of reaction and activation energies at room temperature are collected for several theoretical levels of calculation in Table 6 and compared to the experimental data available. With respect to the heat of reaction, DFT methods are reasonably close, in general, but tend to be outside the error bars of the experimental determination, except in the case of B3PW91 and BMK. Basis set extension is provoking a noticeable effect in the same way as mentioned before. BMK calculations with the more extended basis sets gave an enthalpy of reaction similar to that afforded by the CBS-4M or MP2 methods, while the other CBS and G4 composite methods are nearer to experiment. As expected, the CCSD(T) result is the nearest to the experimental value. [Pg.76]


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