[gmx-users] Force constant defined in flexible TIP3P water model

[Changsun Eun]

Dear Gromacs Users,
 
I have a simple question about the force constants of flexible TIP3P 
water model defined in Gromacs and Amber. Simply, I compared the force 
constants for the bonded interaction between oxygen and hydrogen of 
water and I realized that they seemed to be different (although the 
equilibrium bond length and bond angles are the same as 0.09572 nm and 
104.52 degree, respectivley) The force constant defined in Amber 
(para99.dat or gaff.dat) is 553.0 kcal/mol/(Angstrom^2), which is 
equivalent to 462750 (=553.0*2*4.184*100) kJ/mol/(nm^2) for Gromacs 
format, while the actual force constant in Gromacs (tip3p.itp) is 502416 
kJ/mol/(nm^2).
I am just wondering why they are different. Did I make a stupid mistake 
in this comparison or does this difference not matter in defining 
flexible TIP3P water model? Also, could you anyone explain how they got 
these numbers from the original TIP3P model?
 
Thank you very much for your help.
 
Changsun Eun