### Basic input file

%mem=1000MB %NProcShared=2 #HF/STO-3G SP Comment line 0,1 O -0.0533920156 0.0000000000 -0.0413405155 H -0.0502486311 -0.0000000000 0.9200071213 H 0.8781214711 -0.0000000000 -0.2789994805

Here we see the first line is the requested memory for the calculation. The second line is the number of processors used for the calculation. Based on the theoretical approach being used, some calculations cannot use multiple processors.* If you are using WebMO to build your input files, it is important to specify the number of processors in the job setup screen. Do not edit this in the generate input screen. Due to how WebMO creates the job submission files, it will not submit the correct number of cores.*

Next is the command line known as the route section. Here you input theoretical approach and any other key words. In this example, we are using Hartree-Fock method with the STO-3G basis set. We are performing a single energy point calculation.

Next is a blank line followed by a comment line. Here you put anything.

Following the comment line is another blank line. After the blank line is the charge and multiplicity. In this case, we have a neutral molecule with the multiplicity of 1.

Next is the geometry specification section. For examples on different types of inputs please see Molecule specification page on the Gaussian website.

Finally all Gaussian inputs must end with a blank line.

### Types of calculations

#### Energy Point

#### Geometry Optimization

This keyword requests that a geometry optimization be performed. The geometry will be adjusted until a stationary point on the potential surface is found. Analytic gradients will be used if available. More information can be found on the Gaussian website.

#### Frequency

This calculation type keyword computes force constants and the resulting vibrational frequencies. Intensities are also computed. Raman and IR info is also calculated with the FREQ keyword. If Raman results are not necessary the keyword FREQ(NORAMAN) should be used for bigger calculations to speed up the computation.

If you are having trouble finding a stationary point on a PES, try running a frequency calculation. You must specify a checkpoint file. Then, run a optimization on the same structure as in the frequency and use the force constants stored in the checkpoint to guide the optimization. In order for the optimization to use the force constants you must specify an option for the OPT key word, like *OPT(READFC).*

#### Bond Length Scan

#### Locating Transition States

There are many different ways to locate transition states with Gaussian. A simple TS optimization is shown below.

Another way of finding a transition state is performing a QST2 calculation. Here we are going to include 2 geometries in the input file. The QST2 calculation will then attempt to find a transition state between the two inputed geometries. Here is an example of a QST2 calculation:

If you have a guess to the transition state structure, you may try the QST3 option. In addition to the QST2 calculation guess transition state structure is also included in the input (shown below).

#### IRC

An IRC calculation allows one to map out a reaction pathway by integrating the intrinsic reaction coordinate. In the input, the molecular geometry is of the transition state molecule. An IRC can go in the forward or reverse direction. By default, BOTH reaction pathways are followed. An IRC calculation requires initial force constants. These can be calculated by computing a frequency on the transition state and saving the checkpoint file. Finally the same checkpoint is then used for the IRC calculation with the key word *RCFC*. If a frequency computation was not preformed, one can simply calculate the force constants before the IRC by using the keyword *CALFC*. More info about IRC calculations can be found here

#### NMR

This properties keyword predicts NMR shielding tensors and magnetic susceptibilities using the Hartree-Fock method, all DFT methods and the MP2 method as stated on the NMR page on the Gaussian website.

The Gaussian website also suggests that an NMR calculation be preformed on a *high-quality* structure. The NMR keyword can only be specified with HF, DFT and MP2 methods.

#### Natural Bond Orbital (NBO) version 6

Request NBO6 analysis using the Gaussian keywords of the following forms:

pop=nbo6 -- default NBO6 analysis, no $NBO input pop=nbo6read -- NBO6 analysis with $NBO input pop=nbo6del -- NBO6 analysis with deletions pop=(nbo6,savenbos) -- default NBO6 analysis, save NBOs on the checkpoint file pop=(nbo6,savenlmos) -- default NBO6 analysis, save NLMOs on the checkpoint file pop=(nbo6,savemixed) -- default NBO6 analysis, save sorted NLMOs on the checkpoint file

Note that these keywords are analogous to the pop=nbo, pop=nboread, etc. keywords. Using the plain nbo keywords will invoke the old NBO 3.1 program.

#### Specialized basic sets

Here we will show different ways to use specialized basis sets. First, if the basis set is not a defined basis set for Gaussian one may use the keyword GEN, as shown below.

Here we took the DZP+ basis set from the EMSL Basis Set Exchange website. With the EMSL website, you can specify your desired basis set for each atom and the format of the basis set. In this case, Gaussian94 is the format. (The basis set input has not changed so the Gaussian94 version will work.) For more info visit the Gaussian page on the GEN keyword.

*Extrabasis* keyword: If you want to include additional basis functions such as diffuse functions to a defined basis set, do this by using the extrabasis keyword, shown below.

### Differences Between Gaussian09 and Gaussian16

##### Calculation Defaults

The following calculation defaults are different in Gaussian 16:

- Integral accuracy is 10
^{-12}rather than 10^{-10}in Gaussian 09. - The default DFT grid for general use is UltraFine rather than FineGrid in G09; the default grid for CPHF is SG1 rather than CoarseGrid. See the discussion of the Integral keyword for details.
- SCRF defaults to the symmetric form of IEFPCM [Lipparini10] (not present in Gaussian 09) rather than the non-symmetric version.
- Physical constants use the 2010 values rather than the 2006 values in Gaussian 09.

The first two items were changed to ensure accuracy in several new calculation types (e.g., TD-DFT frequencies, anharmonic ROA). For these reasons, Integral=(UltraFine,Acc2E=12) was made the default. Using these settings generally improve the reliability of calculations involving numerical integration, e.g., DFT optimizations in solution. There is a modest increase in the CPU requirements for these options compared to the Gaussian 09 defaults of Integral=(FineGrid,Acc2E=10).

The G09Defaults keyword sets all four of these defaults back to the Gaussian 09 values. It is provided for compatibility with previous calculations, but the new defaults are strongly recommended for new studies.

##### Default Memory Use

Gaussian 16 defaults memory usage to %Mem=100MW (800MB).

##### TD-DFT Frequencies

TDDFT frequency calculations compute second derivatives analytically by default, since these are much faster than the numerical derivatives (the only choice in Gaussian 09).

#### Reference

Gaussian 16, Revision C.01, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, G. A. Petersson, H. Nakatsuji, X. Li, M. Caricato, A. V. Marenich, J. Bloino, B. G. Janesko, R. Gomperts, B. Mennucci, H. P. Hratchian, J. V. Ortiz, A. F. Izmaylov, J. L. Sonnenberg, D. Williams-Young, F. Ding, F. Lipparini, F. Egidi, J. Goings, B. Peng, A. Petrone, T. Henderson, D. Ranasinghe, V. G. Zakrzewski, J. Gao, N. Rega, G. Zheng, W. Liang, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, K. Throssell, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. J. Bearpark, J. J. Heyd, E. N. Brothers, K. N. Kudin, V. N. Staroverov, T. A. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. P. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, J. M. Millam, M. Klene, C. Adamo, R. Cammi, J. W. Ochterski, R. L. Martin, K. Morokuma, O. Farkas, J. B. Foresman, and D. J. Fox, Gaussian, Inc., Wallingford CT, 2019.

#### Tips

Having problems with gaussview opening checkpoint files? Try these commands below:

formchk file.chk file.fchk sed 's/independent/independant/' file.fchk > file-fixed.fchk

Then just open “file-fixed.fchk” with Gaussview.