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Gn Methods

These method keywords request the following methods for computing very accurate energies:

  • Gaussian-1 (G1) [Pople89, Curtiss90]
  • Gaussian-2 (G2) [Curtiss91]
  • Gaussian-3 (G3) [Curtiss98]
  • Gaussian-4 (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 fourth-generation methods [Curtiss07, Curtiss07a].

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.

Users should generally consider other high accuracy methods before selecting one of these. CBS-QB3 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.


Do only a single-point energy evaluation using the specified compound model chemistry. No zero-point or thermal energies are included.


Perform the frequencies and single-point 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 single-point steps. StartFreq is a synonym for NoOpt.


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/6-31G(d) JobType Temperature=300.0 

0 1

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 18O, 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(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 + ZPE
G2 Energy    Thermal-corrected energy: E = E0 + Etrans + Erot + Evib
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 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
0 1
molecule specification
# G2(Restart,ReadIso) Geom=Check
300.0 1.0
isotope specifications

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