# 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?

• 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. – pentavalentcarbon Mar 19 '18 at 15:25