Force Constants for Angles From Gaussian To AMBER: which value should I put in the matrix equation?

I am trying to make a new force field in AMBER for a novel compound including boron (which does not exist in GAFF). For this, I calculated freq and I found bond force constants but I couldn't find force constants for **angles**.


For example I tested $\ce{H2O}$: I calculated $\ce{H2O}$ frequencies in Gaussian and I got force constants from the log file. In the log file, the $\ce{O-H}$ stretching force constant was $\pu{9.1465 mDyne/\require{mediawiki-texvc}\AA{}}$ and I converted this to approximately $\pu{1315.65 kcal mol^{-1} \AA{}^{-2}}$. In AMBER, $v = 0.5k(l-l\_{o})$, so I multiplied this by 0.5 and I found



```
ow-hw 657.825

```

which is very close to the GAFF2 forcefield:



```
ow-hw  612.98

```

which can be found on [line 87](https://github.com/choderalab/ambermini/blob/master/share/amber/dat/leap/parm/gaff2.dat). However, I couldn't find a way to calculate force constants from Gaussian for angles. In GAFF2 for $\ce{H2O}$, the values are



```
hw-ow-hw   43.276     104.520       AMBER              1    TIP3P_water

```

I searched and I found a lot of answer like [this](https://chemistry.stackexchange.com/a/74923/61506), but in this answer, for the Seminario approach I do not know what value to put in the matrix equation. Could you provide an example on a Gaussian log file?




I really don't know exact answer but I have an idea for you it may help: You can use some "force field developer toolkits". One of them is: VFFDT. It parametrizes (for amber forcefield \*.frmcd files) bond and angle force constant from gaussian out/log file.


 1

First of All thanks for your answer Erdem Caner. After your answer I found VFFDT download link and I created my \*.frcmod file with this program.
Here is the VFFDT link: <https://www.dropbox.com/sh/hlefz8xy8o23q44/AADZGTBN-aJ07flZ-kM0s4U6a?dl=0>
After download the program be careful your OS language must be English. Then run program according to manual which is included in the link I provided. Then you can create your forcefield very easily.
For dihedral parameters use gaussian scan and amber manual7 appendix C (<http://rmni.iqfr.csic.es/HTML-manuals/amber7.pdf> appendix C **page 287**). Then combine all parameters in a \*.frcmod file and then you can run MD simulation for your new molecule.


 1