hello dear users
why we ues the chare of -0.83 for O and 0.415 for H in Tip3p & Tip4p models? (we know that O is 2- & H is 1+)
thanks
Hello Xiang
I believe these values emerge from standard reduction potentials...
The values for these charges (just like most other models) come from the idea that atoms in molecules are partially charged. If we had dissociation, such that we had a hydroxyl ion and a hydrogen (or hydronium ion), then a full charge would be appropriate. However, within a neutral molecule, the concept of a “formal” charge is just that: a formalism, rather than an actual description of the charge.
(Note that in some models, even fully “dissociated” ions, such as Na+, a charge slightly less than +1 is used instead!)
—AEI
hello dear users
why we ues the chare of -0.83 for O and 0.415 for H in Tip3p & Tip4p models?
please note, that TIP4P uses different partial charges (-1.04/+0.52) than TIP3P.
(we know that O is 2- & H is 1+)
no. those are O^2- and H^+. two protons and an oxide anion
don't make a water molecule. there is a chemical reaction
happening which results in a redistribution of electron density.
in classical MD the effect of electrons is integrated out and
what you are left with are "effective" charges that are chose
to mimic the average charge distribution. also we are using
empirical potentials which are parameterized to represent
the real system reasonably well (as far as it is possible with
integrating out a substantial part of the physics that is involved).
cheers,
axel.