Frequency Jobs

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. 

Exploring Chemistry with Electronic Structure Methods 

A frequency job must use the same theoretical model and basis set as produced the optimized geometry. Frequencies computed with a different basis set or procedure have no validity. We U be using the 6-31G(d) basis set for all of the examples and exercises in this chapter. This is the smallest basis set that gives satisfactory results for frequency calculations. 

Note The sample job files for this chapter do not generally include the optimization step. The molecule specifications in these input files have already been set to their optimized values. 

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Including ReadFC is also useful whenever you already have performed a frequency calculation at a lower level of theory. When you have a difficult case and you have no previous frequency job, then CalcFC is a good first choice. CalcAII should be reserved for the most drastic circumstances. 

A frequency job begins by computing the energy of the input structure. It then goes on to compute the frequencies at that structure. Gaussian predicts the frequencies, intensities, and Raman depolarization ratios and scattering activities for each spectral line ... 

The other major properties computed by a frequency job are the polarizability and hyperpolarizability tensors. Normally, the polarizability is printed at the end of the output, just before the archive entry ... 

We are interested in exploring the C3H5F potential energy surface. We will begin hy running optimization and frequency jobs on these three isomers of 1-fluoropropenc... 

All of the optimizations are successful. The frequency jobs for the two forms where the H-C-C-H dihedral angle is 0° produce no imaginary frequencies, and the cis form is lower in energy than the trans form by about 0.63 kcal/mole. 

The frequency job on the middle structure produces one imaginary frequency, indicating that this conformation is a transition structure and not a minimum. But what two minima does it connect Is it the transition structure for the cis-to-trans conversion reaction (i.e. rotation about the C=C bond) ... 

Neither frequency job produces any imaginary frequencies, indicating that both structures are minima. A quick way to check for this is to search the output file for the string imagin such a search indicates that there is no matching line in the file. 

So]ots ji A frequency job on the optimized structure for planar vinyl amine will produce one imaginary frequency. This indicates that it is a transition state, not a minimum... 

The frequency job on this structure will confirm that it is a minimum. We ll consider more of the results of this frequency calculation in the next exercise. ... 

Solution The following table summarizes the results of the frequency jobs on these systems ... 

Predict the zero point or thermal energy by running a frequency job at the optimized geometry, using the same method and basis set. (Note that these two steps maybe run via a single Gaussian job via the Opt Freq keyword.)... 

We will also use the results of the frequency job in the IRC calculation we ll do next. This job will enable us to verify that this transition structure connects the two minima that we think it does, and we use the keyword IRC to request it. By default, the calculation takes 6 steps in each direction, where each step corresponds to a g. jinetry optimization. However, the calculation will stop searching in a given direction once its convergence criteria are met, and an IRC calculation does not necessarily step all the way down to the minimum. 

Note that you will need to compute the thermochemistry of water at both 298.15 and 373 K. Here is the structure of an input file which will allow you to do so without having to run two separate frequency jobs ... 

These values predict a value of -34.0 kcal moT for AH . This value is in excellent agreement with experiment (-34.0 2.0 kcal mol). Note that we didn t need to run a frequency job on Li, but rather computed the enthalpy correction as 2 RT (just a translational component). 

Unfortunately, the frequency job finds an imaginary frequency, indicating that this structure is not a minimum. Here are the displacements corresponding to this frequency ... 

The frequency job for this structure finds no imaginary frequencies, confirming that it is a minimum. 

The frequency job confirms that this structure is a rninimum, finding no imaginary frequencies. Here are the predicted frequencies, compared to the experimental values given earlier ... 

Table 5.3. Effect of basis set and symmetry on times for single point, geometry optimization and geometry optimization + frequencies jobs on acetone, (CH3)2CO...

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