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  (i.e., Lq for "little q"),
CBS-Q , CBS-Q//B3 [92, 93] and CBS-APNO  methods, respectively. No
basis set should be specified with any of these keywords.
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.
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
The CBS-4 model chemistry has also been updated with both the new
localization procedure and improved empirical parameters . 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.
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
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
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.
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
# CBS-4(Restart,ReadIso) Geom=AllCheck Test