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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. 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, 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?

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    $\begingroup$ Hi all, I don't think this should count as homework. Not only because it's unlikely to be asked as homework, but because the translation of quantum chemical calculations to simple force fields is highly desirable. $\endgroup$ – pentavalentcarbon Mar 19 '18 at 15:25
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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.

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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.

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