[gmx-users] Specific question regarding SPC water model

[Justin Lemkul]

On 6/3/13 7:49 PM, tnorgd wrote:
> Dear group,
>
> I am not sure how SPC energy actually works - is there really just one VdW
> interaction - just between two oxygen atoms?
>

Yep.  A lot of force fields work like that.  Given the size of H atoms relative 
to their heavy atom counterparts, the heavy atom to which they are connected 
generally governs the vdW interactions.

> If so, when I place a hydrogen atom of one water molecule at the location of
> the oxygen from another water,
> the two oxygens are separated by the distance of H-O bond length i.e. 1.0A.
> The only VdW interaction is repulsive but finite (and equal ~4E6) but the
> electrostatic between H and O is -Inf = 1/0.0
>
> What's wrong?
>

Nothing.  That makes perfect sense.  You're introducing atomic overlap and thus 
an energetic singularity from the infinite Coulombic attraction.

-Justin

-- 
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Justin A. Lemkul, Ph.D.
Research Scientist
Department of Biochemistry
Virginia Tech
Blacksburg, VA
jalemkul[at]vt.edu | (540) 231-9080
http://www.bevanlab.biochem.vt.edu/Pages/Personal/justin

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