Angle_coeff of H2O in examples/template 109.47 not 104.45 degrees?

in lammps/examples/template/


Angle Coeffs # harmonic
1 100 109.47
2 100 180


it makes sense that the harmonic angle_coef for CO2 is 180, but why is the H2O angle 109.47 not 104.45 degrees ? what am i missing here ??

also how are the K=100 harmonic angle coefficients derived or obtained ?

ps: wow ! i really like the " Your topic is similar to…" feature on this page, never saw this on any forum before. i did check first by searching but just saying this feature is a great idea !

Because this is what the SPC/E water model requires: 8.4.6. SPC water model — LAMMPS documentation

Mind you, the angle in water molecules in the gas(!) phase is 104.52, which is used in TIP3P and TIP4P models. Neither is a perfect representation for liquid water. the TIP class models start with the gas phase geometry and tweak the parameters to approximate liquid and frozen water while the SPC class starts from the fact that condensed water has a tetrahedral environment. As you could see from the (vast amount of) published literature on water, all of them fail to properly represent water and many reparameterizations and tweaks exist for specific conditions.

Those are arbitrary and chosen to be large. For the water it is not relevant all since only the angle is used due to use fix shake.

This is a feature of the Discourse software. See