CBS-Q method

CBS extrapolation 155, 278 CBS methods 10, 96, 155 cost vs. G2 methods 159 CBS-4 method 155 CBS-Q method 155 CCSD keywords 114 CH bond dissociation 186 charge xxxv, xlii, 15, 286 predicted atomic li charge distribution 20 Cheeseman 53 chlorine (atomic) 137, 159 chlorobenzene 165 chromium hexacarbonyl 52 Cioslowski 198 CIS keyword... 

The implementation of Gaussian 98 and the introduction of mnch faster computers coincided with the rise of the Gl, G2 and CBS-Q methods, another milestone in computational chemistry, since chemical accuracy was now available for compounds up to six heavy atoms. Currently, the G3 and CBS-Q methods can treat systems up to ten heavy atoms without too much difficulty, affording bond energies in most cases within l-2kcalmoC accuracy. 

Several empirical corrections are added to the resulting energies in the CBS methods to remove the systematic errors in the calculations (see Table 10). The CBS-Q method contains a two-electron correction term similar in spirit to the higher level correction used in G2 theory, a spin correction term to account for errors resulting from spin contamination in UHF wavefunctions for open-shell systems, and a correction to the sodium atom to account for core-valence correlation effects. The CBS-4 and CBS-q methods also contain a one-electron... 

Since larger basis sets and increased treatment of correlation improve the prediction of acidities, the composite methods potentially offer particular value. In the paper describing the CBS-4 and CBS-Q methods, Petersson notes the excellent agreement with experiment these two methods give for the DPEs of benzene and ethylene. Wiberg examined 16 small acids using the CBS-4, CBS-Q, G2MP2, and... 

Rablen chose fcrf-butanol as the reference system, arguing that C-C bonds are less polar than C-H bonds and thereby eliminates polarization effects that may be inherent in smaller alcohol reference molecules, like methanol or ethanol. The proton exchange shown in Reaction 3.3 defines the relative acidities of acetic acid versus t rt-butanol. Using the CBS-Q method, acetic acid is 27.9 kcal mol" more acidic than t rt-butanol. 

The CBS-Q method is based on the same philosophy of G2 and requires the following calculations [144] ... 

TheCBS-QB3 is a modification of the CBS-Q method, in which steps (b) and (c) are replaced with a B3LYP/6-31G geometry optimization, following the DFT. 

The relative merits of the four most popular G2 and CBS methods are apparent from Table 5.10 (taken from the book by Foresman and Frisch [le]). All four methods are available as a keyword in Gaussian 94 and Gaussian 98. If the test sample is representative, then the CBS-Q method is the most accurate and the G2 the second most accurate (G3 should be somewhat better and faster than G2). As CBS-Q seems to be faster than G2, CBS-Q appears to be the method of choice among these four for most high accuracy energy calculations, unless the system is relatively large, in which case CBS-4, the least accurate of the four, may have to be used. G3 and CBS-Q likely have quite similar errors, and G3 is faster than G2 the times on a Pentium 3... 

Netherlands This study compared MP2/6-311++G(cf,/7), CBS-Q, and B3LYP/6- ill+G d,p) calculations and found that the CBS-Q method gave the best correlation with experimental data. There was also good correlation with MP2 and B3LYP results, which could be used to obtain CBS-Q-like results when the CBS-Q method was not applicable. Table 3.12 gives some of the computed stabilization (-h) and destabilization (-) relative to ethyl and vinyl cations found using isodesmic reactions ... 

Hie CBS-Q, CBS-q, and CBS-4 methods are varieties of CBS (complete basis set) compound methods devised for calculations on molecules containing the atoms H to Ar. These methods use special procedures designed to extrapolate calculated energies to the complete-basis-set limit. Like G2, the CBS methods include several correlated calculations done at a geometry optimized at a lower level of theory. The highest-level calculation used is QCISD(T)/6-31+G (the 6-31+G basis set is a slightly improved version of 6-31G ) in the CBS-Q method, is QCISD(T)/6-31G in the CBS-q method, and is MP4(SDQ)/6-31G in the CBS-4 method. Like G3, the CBS methods contain empirical corrections. For details see J. W. Ochterski, G. A. Petersson, and J. A. Montgomery,/. Chem. Phys., 104,2598 (1996). 

For open-shell species the UFIF method is used, which in some cases suffers from spin contamination. To correct for this an empirical correction based on the deviation of from the theoretical value is added for the CBS-4 and CBS-Q methods, = -0.0092[ - - 1)], where the factor of -0.0092 is derived... 

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