Using freqchk

This is one of the Gaussian utility programs, and can be used to obtain isotopic frequencies using a pre-determined .fchk file.

Step 1

Construct an input file of the type shown below and in this example named cpt-CAS.dat.

cpt-CAS.fchk
n
0
298.0
1.0
n
12 
12 
13
1 
1 
1 
1 
1 
12 
1
1 
n

The integers 12 ... 1 are the atomic masses of the atoms specified in the .fchk file in the same order that they appear in the input file used to run the Gaussian calculation

Step 2

Invoke the command

freqchk <cpt-CAS.dat >cpt-CAS-13C3.log

This writes out a partial Gaussian like log file from which the isotopic frequencies can be obtained. This file does NOT load into eg Gaussview.