CBS-APNO

There are other defined CBS methods, including the very accurate and very expensive CBS-APNO, but we will be focusing on CBS-4 and CBS-Q. 

There are several different CBS methods, each having its own set of prescriptions and resulting computational cost and accuracy, they are known under the acronyms CBS-4, CBS-q, CBS-Q and CBS-APNO. As an explicit example, we will take the CBS-Q model, " which is computationally similar to the G2(MP2) method. 

We will concentrate on Gaussian-type and CBS methods, because these have been the most widely-used and have thus accumulated an archive of results, are the most accessible, and because several versions of them are available. However, there are other high-accuracy multistep methods, such as the Weizmann procedures of Martin and de Oliveira, W1 and W2 [188], and of Boese et al., W3 and W4 [189], which like the CBS methods are based on basis set extrapolation. W1 and W2 have a mean absolute deviation of about 1 kJ mol 1 (not 1 kcal mol-1), and incorporate relativistic effects, and W2 has no empirical parameters, unlike the Gaussian and CBS methods. W3 and W4 methods have similar errors to W1 and W2, and the authors speculate on the reasons for the obstinate 0.1 kcal/mol barrier . These very accurate methods are still limited molecules of about or less than the reach of CBS-APNO. 

Of the Gaussian-type and CBS methods, for high accuracy on very small molecules CBS-APNO is the appropriate choice, and for large molecules the choice falls on CBS-4M with the acceptance of the possibility of moderately large errors. For intermediate size molecules the best choice is probably between G3 (MP2) and CBS-QB3. Recall that G3(MP2) is much faster than G3, with little loss of accuracy in most cases note too that G4(MP2) will compete with and when widely... 

Table 5.10 Comparison of speed and ability to handle molecular size for four popular high-accuracy multistep methods G3(MP2), CBS-4M, CBS-QB3, and CBS-APNO Time (h) for less than 1 h h (min)...

Table 7.4 Reaction enthalpies (kJ mol-1), calculated with three functionals and two basis sets, 6-31G and 6-311++G(2df, 2p), and with three high-accuracy methods (but CBS-APNO is unable to handle Cl species). The calculated reaction enthalpies follow from the calculated 298 K product enthalpies minus the reactant enthalpies...

Term (2) calculated by the high-accuracy multistep CBS-APNO method (Section 5.5.2.2b) was 341.2 kcal mol-1 or 1426 kJ mol-1. The Sackur-Tetrode equation for the gas-phase entropy of the proton was mentioned in this regard, but in fact the algorithm automatically handles this. 

Term (3) is the gas-phase free energy of deprotonation of the reference acid CH3COOH this can be calculated accurately with the high-level CBS-APNO, giving = [-228.500394 - 0.010000] - [-229.053416] = 0.543022 hartees = 1425.7 kJ mol-1. 

Compare this with a direct CBS-APNO calculation on CH2FCOOH ... 

Canonical momentum operator, 248 Car and Parrinello (CP) method, 388 CASMP2, CASPT2 methods, 132 CASVB method, 202 Cavity, in reaction field models, 393 CBS-4, CBS-q, CBS-Q and CBS-APNO methods, 167... 

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