method keywords request the Gaussian-1 (more colloquially known as G1) [80,81],
Gaussian-2 (G2) , and Gaussian-3 (G3) 
methods for computing very accurate energies. G2MP2 requests the modified
version of G2 known as G2(MP2), which uses MP2 instead of MP4 for the basis set
extension corrections , and is nearly as accurate
as the full G2 method at substantially reduced computational cost. G3MP2
requests the similarly modified G3(MP2) method .
The G3 variants using B3LYP structures and frequencies 
are requested with the G3B3 and G3MP2B3 keywords.
All of these
methods are complex energy computations involving several pre-defined calculations
on the specified molecular system. All of the distinct steps are performed automatically
when one of these keywords is specified, and the final computed energy value is
displayed in the output. No basis set keyword should be specified with these keywords.
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.
specify alternative isotopes for these 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] Must be real numbers.
isotope mass for atom 1
isotope mass for atom 2
isotope mass for atom n
temp, pressure, and scale are the desired temperature, pressure,
and an optional scale factor for frequency data when used for thermochemical analysis
(the default value for the corresponding model is used if scale is omitted
or set to 0.0); these values must be real numbers. 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).
Resume a partially-completed calculation
from its checkpoint file. When used in combination with ReadIso, this option
allows for the rapid computation of the energy using different thermochemistry
parameters and/or isotope selections.
Assume that the
specified checkpoint file contains the results of a Hartree-Fock frequency calculation
at the HF/6-31G* optimized structure, and begins the G2 calculation from that
point (implies Geom=AllCheck).
Summary Output. After all of the output for the component job steps, Gaussian
prints a table of results for these methods. Here is the output from a G2
Temperature= 298.150000 Pressure= 1.000000
E(ZPE)= .020511 E(Thermal)= .023346
E(QCISD(T))= -76.276078 E(Empiric)= -.024560
DE(Plus)= -.010827 DE(2DF)= -.037385
G1(0 K)= -76.328339 G1 Energy= -76.325503
G1 Enthalpy= -76.324559 G1 Free Energy= -76.303182
E(Delta-G2)= -.008275 E(G2-Empiric)= .004560
G2(0 K)= -76.332054 G2 Energy= -76.329219
G2 Enthalpy= -76.328274 G2 Free Energy= -76.306897
The temperature and pressure appear first, followed by the various components
used to compute the G2 energy. The output concludes with the G2 energy at 0 K
and at the specified temperature (the latter includes a full thermal correction
rather than just the zero-point energy correction), and (in the final output line)
the G2 theory predictions for the enthalpy and Gibbs free energy (both computed
using the thermal-corrected G2 energy). (Note that the same quantities predicted
at the G1 level are also printed in this summary section.)
The energy labels
thus have the following meanings (G2 is used as an example):
G2 (0 K)
Zero-point-corrected electronic energy: E0 = Eelec +
Thermal-corrected energy: E = E0 + Etrans
+ Erot + Evib
using the G2 predicted energy: H = E + RT
G2 Free Energy
Free Energy computed using the G2 predicted energy: G = H - TS
the Calculation at a Different Temperature. The following two-step job illustrates
the method for running a second (very rapid) G2 calculation at a different temperature.
This job computes the G2 energy at 298.15 K and then again at 300 K:
# G2 Test
G2 on formaldehyde
# G2(Restart,ReadIso) Geom=Check
Repeat at 300 K