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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.

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  • $\begingroup$ I haven't attempted to validate this post yet, but it is suggested in the following link that if you include #p freq=intmodes, it should return the internal hessian under "Internal force constants" in the log file. researchgate.net/post/… $\endgroup$ – Tyberius Dec 6 '17 at 18:49
  • $\begingroup$ I have tried that, you do get the internal modes in the log file if the molecule is small. But for bigger molecules it stops printing either the cartesian or the internal hessian in the log file. Though the cartesian hessian can be found in the checkpoint file. $\endgroup$ – ace_101 Dec 6 '17 at 19:14
  • $\begingroup$ It is highly discouraged to cross-post the same question to different sites (not only within our network). $\endgroup$ – Martin - マーチン Dec 19 '17 at 8:01
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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, too.

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