Hi,
I am trying to make a *.dat file manually and I calculated force constant with gaussian. In gaussian force const. units are:mdyne/A but in *.dat files it is: kcal/molA^2 after that I found on the internet 1 mdyne/A = 143.836 kcal/molA^2 , but when I converted my results with this equation I got some error. So, I researched literature and I found this article: https://sites.smu.edu/dedman/catco/publications/pdf/JMolModel_6_396_2000.pdf
In this article, table 1, for CH4 bond force constant is (approximately) :5,4 mdyne/A = 776 according to the equation but when I looked the gaff2.dat I found that :c3-h1 force constant is: 375.92 which is almost twice of literature.
and my quesition is: What is the cause of this error and how can I fix it?
Thank you
In the paper you mentioned, eqs 14 defined the potential energy for bond stretching to be:
V=(1/2) k (r - r0)^2
But in amber's manual(http://ambermd.org/doc12/Amber17.pdf), eqs14.1 (page 243) gives the form of potential energy for bond stretching:
V' = k' (r - r0)^2
So, the Amber force constant k' = (1/2)k
Dear Shaou and Stephane
Thank you so much for your answers. It really helped me. Thank you so much again.
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