These method keywords request the following methods for computing very accurate energies:
 Gaussian1 (G1) [Pople89, Curtiss90]
 Gaussian2 (G2) [Curtiss91]
 Gaussian3 (G3) [Curtiss98]
 Gaussian4 (G4) [Curtiss07]
 G2MP2 requests the modified version of G2 known as G2(MP2), which uses MP2 instead of MP4 for the basis set extension corrections [Curtiss93] and is nearly as accurate as the full G2 method at substantially reduced computational cost.
 G3MP2 requests the similarly modified G3(MP2) method [Curtiss99].
 The G3 variants using B3LYP structures and frequencies [Baboul99] are requested with the G3B3 and G3MP2B3 keywords.
 G4 and G4MP2 request the fourthgeneration methods [Curtiss07, Curtiss07a].
All of these methods are complex energy computations involving several predefined 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.
Users should generally consider other high accuracy methods before selecting one of these. CBSQB3 is equally accurate and significantly faster, while W1U is more accurate (but slower).
Either of the Opt=Maxcyc=n, QCISD=Maxcyc=n, or CCSD=Maxcyc=n keywords may be used in conjunction with any of the these keywords to specify the maximum number of optimization, QCISD, or CCSD cycles, respectively.
SP
Do only a singlepoint energy evaluation using the specified compound model chemistry. No zeropoint or thermal energies are included.
NoOpt
Perform the frequencies and singlepoint energy calculation for the specified model chemistry at the input geometry. Freq=TProjected is implied. This option is not meaningful or accepted for methods such as G1, which use different geometries for the frequencies and the singlepoint steps. StartFreq is a synonym for NoOpt.
ReadIsotopes
This option allows you to specify alternatives to the default temperature, pressure, frequency scale factor and/or isotopes—298.15 K, 1 atmosphere, no scaling, and the most abundant isotopes (respectively). It is useful when you want to rerun an analysis using different parameters from the data in a checkpoint file.
Be aware, however, that all of these can be specified in the route section (Temperature, Pressure and Scale keywords) and molecule specification (the Iso parameter), as in this example:
#T Method/631G(d) JobType Temperature=300.0 …
…
0 1
C(Iso=13)
…
ReadIsotopes input has the following 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 unscaled). 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 isotopic mass (e.g., 18 specifies ^{18}O, and Gaussian uses the value 17.99916).
Calculation 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 calculation:
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(DeltaG2)= .008275 E(G2Empiric)= .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 zeropoint energy correction), and (in the final output line) the G2 theory predictions for the enthalpy and Gibbs free energy (both computed using the thermalcorrected 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) 

Zeropointcorrected electronic energy: E_{0} = E_{elec} + ZPE 
G2 Energy 

Thermalcorrected energy: E = E_{0} + E_{trans} + E_{rot} + E_{vib} 
G2 Enthalpy 

Enthalpy computed using the G2 predicted energy: H = E + RT 
G2 Free Energy 

Gibbs Free Energy computed using the G2 predicted energy: G = H – TS 
Rerunning the Calculation at a Different Temperature. The following twostep 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:
%Chk=formald
# G2 Test
G2 on formaldehyde
0 1
molecule specification
Link1
%Chk=formald
%NoSave
# G2(Restart,ReadIso) Geom=Check
300.0 1.0
isotope specifications