Calculating Zero Point Energy (ZPE) corrections and Thermodynamic Properties

When you run a regular Jaguar calculation, the molecule is being calculated in the gas phase at absolute zero. To calculate the ZPE correction and thermodynamic properties at a particular temperature (usually 298 K), we need to calculate the frequencies of the molecule.

Here's what you do after your gas phase calculation is complete.
1. Copy the .01.in file to a new input file. You will then make the following changes to the &gen section of your new input file (call it filename_afreq.in or something like that, I use "afreq" to refer to "analytical frequencies")
2. Remove the keyword igeopt=1 (since you don't need to optimize the geometry further, and it would take a l-o-n-g time if you did)
3. Add the keyword ifreq=1
4. I usually remove ip11=2 and icfit=1 and any other "output" related keywords so the output file won't be so big and unwieldy.

There are usually three energy numbers (in hartrees) at the end of the output file
Utot - includes ZPE corrections
Htot - includes both ZPE corrections and enthalpy corrections to 298 K
Gtot - includes ZPE corrections, enthalpy corrections and free energy corrections at 298 K

To calculate the ZPE correction, you can compare it to the electronic energy, i.e., take the difference between the two numbers.

The enthalpy correction is Htot - Utot
The entropy correction (T*dS) is Gtot - Htot
The free energy correction is Gtot - Utot

Quick & Dirty Frequency Calculation using the Available Hessian
(Not publishable)
If you wanted a quick look to see if you have any negative frequencies, you can just diagonalize the available Hessian from your finished geometry or transition state optimization.

Here's what you do after your gas phase calculation is complete.
1. Copy the .01.in file to a new input file. You will then make the following changes to the &gen section of your new input file (call it filename_qfreq.in or something like that, I use "qfreq" to refer to "quick frequencies")
2. Remove the keyword igeopt=1 (since you don't need to optimize the geometry further, and it would take a l-o-n-g time if you did)
3. Add the keyword ifreq= -1 (That's MINUS one!)
4. I usually remove ip11=2 and icfit=1 and any other "output" related keywords so the output file won't be so big and unwieldy.