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.
hi
from gaussian09 get only natural force constans depends on number of vibrations
1. Use #p freq=intmodes
2. In LOG file, you can find them under "Internal force constants".
3. Alternatively, use "formchk -3 xxx.CHK xxx.fchk" to generate a FCHK file. The flag "-3" is essential to write the internal ones into FCHK file. In FCHK file, you can find them under "Internal Force Constants".
4. If you'd like to modify internal coordinates, use "#p freq=(intmodes,modredundant)".
The former one (LOG file) is human friendly and the latter is programming friendly.
For the latter one, I've written some simple codes to deal with lower triangular matrices. Have a look at https://github.com/ruixingw/rxcclib/blob/dev/utils/my/LTMatrix.py
Thanks a lot, I was not using the -3 while converting the .chk to .fchk file.
Hi Ruixing
I had one more small doubt. What are the units of the internal hessian. I know for distances between atoms it is Ha/(bohrs)^2. But what are they for the angles and the dihedrals?
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