How to get force constants in internal coordinates in Gaussian? I have tried quite a few different ways to get Gaussian to print the force constants in internal coordinates. If the size of the molecule is small then it prints both the internal hessian and Cartesian hessian in the log file on using Iop(7/32=5), it outputs only the Cartesian hessian without the Iop command. But if the size of the molecule is large then it stops printing the hessian in the .log file. Though the Cartesian hessian is always printed in the checkpoint file. How do I get the internal hessian? I am also using #p to get Gaussian to write everything in the log file.
You have to use freq=intmodes and use the command

formchk -3 xxxx.chk xxxx.fchk 


to convert the binary checkpoint file to a formatted checkpoint file, which consists of text. The -3 requests the version 3 of formatted checkpoint files, which support all features of Gaussian 09 and 16.

Source: Ruixing Wang's answer on Researchgate, where I asked the same question.