The Route Section

These items form the route section, the title section, and the molecule specification section of the input file, respectively. We ll look at each of them again briefly as we set up an input file for an energy calculation on formaldehyde. 

The route section of a Gaussian input file specifies the kind of job you want to run as well as the specific theoretical method and basis set which should be used. All of these items are specified via keywords. Recall that the first line of the route section begins with a sign (or T to request terse output). 

Here are some other useful keywords for single point energy calculations (and other types of jobs as well)  

Test Prevents Gaussian from entering this job s results into the site archive. 

Pop=Reg Displays highest five occupied and lowest five virtual molecular orbitals and other information not included in the output by default. Use PopsFull to display all orbitals. 

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The first line in File 9-1 is the route section calling for a PM.3 optimization. The next three lines are a blank line, a program label (human input not read by the system), and a blank line. The input (J I indicates that the charge on the molecule is 0 and the spin multiplicity is I (paired electrons). The starting geometry is given in... 

To go from a semiempirical calculation in the GAUSSIAN implementation (File 9-1) to an ab initio calculation, one need only change PM3 in the route section of the input file to sto-3g for a single point calculation or sto-3g opt for an optimization. We have made this change in File 10-1 along with the substitution of h for f in the second line of the geometry section to calculate the molecular... 

The keyword is in the route section, line 1 of the input file. Lines 2, 3, and 4 are blank, comment, blank, respectively. Line 5 designates a charge of 1 and a spin multiplicity of 2 (a doublet). Line 6 specifies one atom as hydrogen, and line 7 specifies the second atom as hydrogen, attached to atom 1 at a distance of 1.32 A(2.49 bohr). Among several G2 energies printed out in about the last 25 lines of output are... 

Compare the energy found using 6-3IG in the route section of the Hj file in the... 

GAUSSIAN implementation with the result found using MP2/6-31G in the route section. 

Here we give the molecule specification in Cartesian coordinates. The route section specifies a single point energy calculation at the Hartree-Fock level, using the 6-31G(d) basis set. We ve specified a restricted Hartree-Fock calculation (via the R prepended to the HF procedure keyword) because this is a closed shell system. We ve also requested that information about the molecular orbitals be included in the output with Pop=Reg. 

In a higher level energy calculation, values for the energy computed using the more accurate procedure appear shortly after the Hartree-Fock energy. Here is the output from a formaldehyde calculation done at the MP2 level (RMP2 replaces RHF in the route section ... 

The Pop=Reg keyword in the route section requested data about the molecular orbitals be included in the output. They appear at the beginning of the population analysis section (output is shortened) ... 

FuUerene compounds have receieved a lot of attention in recent years. In this exercise we predict the energy of Cgg and look at its highest occupied molecular orbital, predicted at the Hartree-Fock level with the 3-21G basi set. Include SCF=1ight in the route section of the job. 

The Opt keyword in the route section requests a geometry optimization, using the basis set and level of theory specified by the other keywords. 

This optimization was very close to a minimum at step two, but then it moved away from it again in subsequent steps. Merely increasing the number of steps will not fix the problem. A better approach is to start a new optimization, beginning with the structure corresponding to step 2 and including the CalcFC option in the route section. 

Q the STO-3G or 3-21G basis set. Include SCF=NoVbrAcc in the route section of your job... 

We have used the OptsAddRedundant keyword in the route section of each job, along with the following additional input sections following the molecule specification (which reflect the atom numbering for our molecule specifications—yours may differ) ... 

Including the Freq keyword in the route section requests a frequency job. The other sections of the input file are the same as those we ve considered previously. 

Because of the nature of the computations involved, firequency calculations are valid only at stationary points on the potential energy surface. Thus, frequency calculations must be performed on optimized structures. For this reason, it is necessary to run a geometry optimization prior to doing a frequency calculation. The most convenient way of ensuring this is to include both Opt and Freq in the route section of the job, which requests a geometry optimization followed immediately by a firequency calculation. Alternatively, you can give an optimized geometry as the molecule specification section for a stand-alone frequency job. 

Here is the route section from the input file ... 

You can specify a different temperature, pressure, and/or set of isotopes for the thermochemical analysis by specifying the Readlsolopes option to the Frecj keyword in the route section. Values for all parameters must then be specified in a separate input section following the molecule specification—and separated from it by a blank line. 

If you begin the route section with P rather than T, then additional information is printed at various points in the job. One of these items is a display of the polarizability and hyperpolarizability tensors much earlier in the output, just prior to the frequency results ... 

Run a single-point energy calculation on methanol using the HF/6-31++G(d,p) model chemistry, including the GFPrint and GFinput keywords in the route section which request that the basis set information be included in the output file (in tabular and input format, respectively). Examine the basis set output and identify its main components. 

Compute the isotropic hyperfine coupling constant for each of the atoms in HNCN with the HF, MP2, MP4(SDQ) and QCISD methods, using the D95(d,p) basis set Make sure that the population analysis for each job uses the proper electron density by including the Density=Current keyword in the route section. Also, include the 5D keyword in each job s route sectionfas was done in the original study). 

Beginning with the final optimized structure from step 1, obtain the fii equilibrium geometry using the fuU MP2 method—requested with t MP2(Full keyword in the route section—which includes inner sh electrons. The 6-31G(d) basis set is again used. This geometry is used 1 all subsequent calculations. 

This input file requests a potential energy surface scan for CH by including the Scan keyword in the route section. The variables section of the molecule specification uses an expanded format ... 

In Gaussian, a reaction path calculation is requested with the IRC keyword in the route section. Before you can run one, however, certain requirements must be met. An IRC calculation begins at a transition structure and steps along the reaction path a fixed number of times (the default is 6) in each direction, toward the two minima that it connects. However, in most cases, it will not step all the way to the minimum on either side of the path. 

For our initial geometry for the transition structure, we ll detach one hydrogen from the carbon and increase the O-C-H bond angle. We specified the Opt=(TS,CalcFC) keyword in the route section, requesting an optimization to a transition state. The CalcFC option is used to compute the initial force constants, a technique which is generally helpful for transition state optimizations. We ve also included the Freq keyword so that a frequency calculation will automatically be run at the optimized geometry. 

In order to save computation time, set up the second and subsequent jobs to extract the electron density from the checkpoint file by using the Geom=Checlcpoint and Densiiy=(Checkpoint/AP2) keywords in the route section. You will also need to include Den iiy=MP2 for the first job, which specifies that the population analysis should be performed using the electron density computed at the MP2 level (the default is to use the Hartree-Fock density). 

Run your study at the Hartree-Fock level, using the 6-31+G(d) basis set. Use a step size of 0.2 amu bohr for the IRC calculation (i.e., include IRC=(RCFC, StepSize=20) in the route section). You will also find the ColcFC option helpful in the geometry optimizations. 

Cl density method, which uses analytic derivatives of the wavefunction to compute the dipole moments, resulting in much more accurate predictions, as is illustrated in this case. You can request the Cl density by including either DensityaCI or DensityaCurrenI in the route section of a Cl-Singles calculation, n... 

Benzene will dearly illustrate this effect. Compare the first six excited states, as predicted using the 6-31G and 6-31-t-G basis sets. When setting up the route section for these jobs, include the NSfate =8 option. Although we are only looking for six... 

Include the keywords IOP(9/40=3) and Pop=Full in the route section of your jobs. The latter requests that all molecular orbitals (occupied and virtual) be included in the population analysis, while the former specifies that all wavefunctioii coefficients greater than 0.001 be included in the excited state output (by default, only those greater than 0.1 are listed). 

You can also use the orbital listing labeled Orbital Syiametries if you did not include PopaFull in the route section. 

Accordingly, we will swap orbitals 6 and 13. We define the active space to comprise these orbitals by using the Gues =Alter keyword in the route section for the first job step in our second calculation series ... 

Include P in the route section of the final job. The various states will be described in the output following the final CAS iteration and introduced by the line ... 

Locate the conical intersection by running a CAS Opl=Conical jobs. Include NoSymm and IOp(1/8=S) in the route section. 

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