G1, G2 and G2MP2 Keywords
Last Update: 6/26/2001

These method keywords request the Gaussian-1 (more colloquially known as G1) [70-71] and Gaussian-2 (G2) [72] 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 [73], and is nearly as accurate as the full G2 method at substantially reduced computational cost. 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.

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. Note, however, that they may not be combined with the Restart option.

ReadIsotopes
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

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 value of 1/1.12 (approx. 0.8929) 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 98 uses the value 17.99916).

Restart
Resume a partially-completed calculation from its checkpoint file. When used in combination with the ReadIso option, this option allows for the rapid computation of the energy using different thermochemistry parameters and/or isotope selections.

StartMP2
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).

Availability: These methods are available for systems comprised of atoms through chlorine.

Calculation Summary Output. After all of the output for the component job steps, Gaussian 98 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 (0 K): Zero-point-corrected electronic energy: E₀ = E_elec + ZPE

G2 Energy: Thermal-corrected energy: E = E₀ + 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 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:

%Chk=formald
# G2 Test

G2 on formaldehyde

0 1
molecule specification

--Link1--
%Chk=formald
%NoSave
# G2(Restart,ReadIso) Geom=All Test

Repeat at 300 K

0,1

300.0 1.0  
isotope specifications

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