Previous Thread
Next Thread
Print Thread
Joined: Nov 2005
Posts: 10
M
Forum Member
OP Offline
Forum Member
M
Joined: Nov 2005
Posts: 10
In the CHARMM version of TIP3P, there are LJ parameters for both hydrogen and oxygen and usuall mixing rules are used to obtain cross LJ interaction with other molecules.

However, if I were to use the original TIP3P, there is no LJ interaction for the hydrogen atom. I wonder what is keeping the hydrogen atom on the water from collapsing into other atoms of opposite charges?

Joined: Sep 2003
Posts: 4,861
Likes: 10
Forum Member
Online Content
Forum Member
Joined: Sep 2003
Posts: 4,861
Likes: 10
LJ on water oxygen


Lennart Nilsson
Karolinska Institutet
Stockholm, Sweden
Joined: Sep 2003
Posts: 8,623
Likes: 24
rmv Online Content
Forum Member
Online Content
Forum Member
Joined: Sep 2003
Posts: 8,623
Likes: 24
Indeed, if you compare the O atom VDW radius, the H atom VDW radius, and the O-H bond length, you'll find that the H atom VDW sphere is actually completely inside the O atom sphere.


Rick Venable
computational chemist

Joined: Dec 2005
Posts: 1,535
Forum Member
Offline
Forum Member
Joined: Dec 2005
Posts: 1,535
And because the H's VdW sphere is entirely inside the O's VdW sphere, the L-J parameters on the H have a negligible effect on bulk phase properties. They're there only for practical purposes. Specifically, without them (and if SHAKE is not in use), it's not unthinkable that during a simulation with an anionic solute, a hydrogen atom would get so close to the solute's negatively charged center X that the H-X electrostatic attraction would become stronger than the O-H bond stretching potentials, causing the simulation to crash because of electrostatic catastrophe.
Edit 2: this is somewhat of a moot point, however, because TIP3P was originally conceived as a rigid water model, so SHAKE should be in use, which rules out the scenario outlined above. In theory, it's still possible for a negative center to penetrate the oxygen's L-J sphere far enough to make the electrostatic energy go to minus infinity, thereby overriding the L-J repulsion, but I wonder what the odds are that this would happen in practice.

Edit 1: A similar question has been answered before. I'm starting to thing we should have a "CHARMM FAQ" somewhere... frown

Last edited by Kenno; 03/17/10 08:09 PM.
Joined: Sep 2003
Posts: 8,623
Likes: 24
rmv Online Content
Forum Member
Online Content
Forum Member
Joined: Sep 2003
Posts: 8,623
Likes: 24
It should be noted that TIP3P is a rigid model, so SHAKE should be used to conform fully to the model. Bulk phase properties change if you omit the SHAKE command.


Rick Venable
computational chemist

Joined: Dec 2005
Posts: 1,535
Forum Member
Offline
Forum Member
Joined: Dec 2005
Posts: 1,535
I was just thinking the same thing when re-reading my own post, hence the edit. grin


Moderated by  alex, lennart, rmv 

Link Copied to Clipboard
Powered by UBB.threads™ PHP Forum Software 7.7.5
(Release build 20201027)
Responsive Width:

PHP: 7.3.31-1~deb10u1 Page Time: 0.011s Queries: 26 (0.007s) Memory: 0.7542 MB (Peak: 0.8124 MB) Data Comp: Off Server Time: 2022-12-04 21:46:57 UTC
Valid HTML 5 and Valid CSS