[gmx-users] Specific question regarding SPC water model

Justin Lemkul jalemkul at vt.edu
Tue Jun 4 02:04:01 CEST 2013

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 A. Lemkul, Ph.D.
Research Scientist
Department of Biochemistry
Virginia Tech
Blacksburg, VA
jalemkul[at]vt.edu | (540) 231-9080


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