Gaussian 03 Online Manual
Last update: 2 October 2006


These method keywords specify the various Complete Basis Set (CBS) methods of Petersson and coworkers for computing very accurate energies [87, 88, 89, 90, 91, 92, 93, 327]. The keywords refer to the modified version of CBS-4 [91, 92], CBS-q [89] (i.e., Lq for "little q"), CBS-Q [91], CBS-Q//B3 [92, 93] and CBS-APNO [91] methods, respectively. No basis set should be specified with any of these keywords.

These methods are complex energy computations involving several to many pre-defined calculations on the specified system. All of these distinct steps are performed automatically when one of these keywords is specified, and the final computed energy value is displayed in the output.

Either of the Opt=Maxcyc=n or QCISD=Maxcyc=n keywords may be used in conjunction with any of the these keywords to specify the maximum number of optimization or QCISD cycles, respectively.

You should specify alternative isotopes for CBS jobs using the standard method. However, the ReadIsotopes option is retained for rerunning completed calculations under different conditions (see the examples).

Specify alternate temperature, pressure, and/or isotopes (the defaults are 298.15 K, 1 atmosphere, and the most abundant isotopes). This information appears in a separate input section having the format:

temp pressure [scale]              Values must be real numbers. 
isotope mass for atom 1
 isotope mass for atom 2
 isotope mass for atom n 

where temp, pressure, and scale are the desired temperature, pressure, and an optional scale factor for frequency data when used for thermochemical analysis (the default is the value defined by the selected method). The remaining lines hold the isotope masses for the various atoms in the molecule, arranged in the same order as they appeared in the molecule specification section. If integers are used to specify the atomic masses, the program will automatically use the corresponding actual exact mass (e.g., 18 specifies O18, and Gaussian uses the value 17.99916).

Restart from the checkpoint file from a previous CBS calculation. If the previous calculation did not complete, it will be completed.

Energies only. CBS-4M, CBS-Lq, CBS-Q and CBS-QB3 are available for first and second row atoms; CBS-APNO is available for first row atoms only.

The CBS-4 model chemistry has also been updated with both the new localization procedure and improved empirical parameters [92]. The new version, CBS-4M, (M referring to the use of Minimal Population localization) is recommended for new studies; the CBS-4O keyword requests the earlier parametrization.

The output from each step of a CBS method calculation is included in the output file. The final section of the file contains a summary of the results of the entire run.

CBS Summary Output. Here is the output from a CBS-Q calculation on CH2 (triplet state):

Complete Basis Set (CBS) Extrapolation: 
G. Petersson and M. A. Al-Laham, JCP 94, 6081 (1991) 
G. Petersson, T. Tensfeldt & J. A. Montgomery, JCP 94, 6091 (1991) 
additional references ... 

Temperature=    298.150000     Pressure=            1.000000 
E(ZPE)=            .016835     E(Thermal)=           .019690 
E(SCF)=         -38.936531     DE(MP2)=             -.114652 
DE(CBS)=          -.011929     DE(MP34)=            -.018702 
DE(QCI)=          -.002781     DE(Int)=              .004204 
DE(Empirical)=    -.005891 
CBS-Q (0 K)=    -39.069447     CBS-Q Energy=      -39.066592 
CBS-Q Enthalpy= -39.065647     CBS-Q Free Energy= -39.043444

The temperature and pressure are given first, followed by the components terms of the CBS-Q energy. The second-to-last line gives the CBS-Q energy values (reading across): at 0 K and at the specified temperature (298.15 K by default). The final line gives the CBS-Q enthalpy (including the thermal correction for the specified temperature) and the Gibbs free energy computed via the CBS-Q method (i.e., the CBS-Q energy including the frequency job free-energy correction). All of the energies are in hartrees.

Rerunning the Calculation at a Different Temperature. The following two-step job illustrates the method for running a second (very rapid) CBS calculation at a different temperature. This job computes the CBS-4 energy at 298.15 K and then again at 300 K:

# CBS-4 Test 

CBS-4 on formaldehyde 

0 1 
molecule specification 

# CBS-4(Restart,ReadIso) Geom=AllCheck Test 

300.0 1.0 
isotope specifications