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. 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?
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.