Subject: Re: CCL:stable Ih ice

From: James Givens <[email-masked]>
Date: Tue, 23 May 2000 16:41:09 -0700

Richard Gillilan wrote (in part):
> 
> I was recently asked to generate an animation sequence of
> a solid melting into a fluid and then vaporizing. Since it
> was for purposes of illustration only (a low-level
> educational film) we did not need a great deal of
> accuracy in the model, just the basic behavior.

There is a nice Quicktime animation on H20 transitions at:
http://www.molecules.com/movies.shtml

This only applies to your project in general, but I thought it 
may help your efforts as background/example material.

Sincerely,
-- 
James Givens
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Molecular Arts Corporation            | <[email-masked]>
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From: Keith Refson <[email-masked]>
Date: Wed, 24 May 2000 10:04:48 +0100 (BST)

Richard Gillilan writes:

 > I constructed a slab of Ih ice from the crystal structure.
 > Because the structure is space and time averaged, or perhaps
 > because the protons hop around a lot, the P63/mmc space group
 > symmetry operators place 24 water molecules in the unit cell in
 > 4 overlapping groups of 6. There are only 4 unique Oxygen positions,
 > but each has 4 unique protons, so there appear to be clashes between
 > neighboring protons. Sorry, this is hard to describe in words.

That's not too bad a discription of the proton disorder characteristic 
of Ice Ih.  Pauling expressed it a little more concisely with his "ice 
rules"

1. H2O Molecules are intact, neglecting the small fraction of ionized
   molecules.

2. Each H2O molecule is oriented so that its two O-H bonds are
   directed approximately towards two of the four nearest neighbouring 
   oxygen atoms.

3. The orientation of adjacent water molecules are such that only one
   hydrogen atom lies approximately along the axis between adjacent
   oxygen atoms.

 > I can simply choose four unique H2O molecules per unit cell
 > and get a  nice lattice with no clashes, which is what I did.

You have made an ordered variant of the ice Ih lattice.  It will have
a large dipole moment so don't expect to get the melting temperature
too accurately as the dipole energy will cause significant bias
depending on your electrostatic boundary conditions.

Since you have more than one unit cell, it may be worth creating more
disorder between the cells, or at least constructing an ordered state
with no dipole.

 > When I minimize a block of ice, I was surprised to find that the
 >  the crystal structure appears to collapse .... or at least there is
 > no minimum near the ice structure.  I'm using a simple pairwise
 > VDW+point charge model.

Is this something like the TIP4P or SPC/E models?  Either of those
should give a good ice structure, and be suitable for your movie.

 > 
 > Q1:  Is a more advanced potential model (say with explicit H bonds)
 >      needed to have stable Ih ice, or is there a good set of point
 >      charges and VDW parameters that will work?

Both TIP4P, SPC/E and a number of simple variants are rigid-body
models with point charges and Lennard-Jones interactions between the
oxygens.   You can't easily simulate any of the TIP4P family using a
constraints MD program because one of the charge sites has no mass,
but a true rigid body code will have no problems with this.

 > 
 > Q2:  The 24 symmetry operators fall into 4 groups of 6, each of which
 >         has the same z-value in common. For example:
 > 
 >           -x+y,    -x, -z+1/2
 >           -y    ,    -x, -z+1/2
 >           -x+y,    -y, -z+1/2
 >             x    ,  x-y, -z+1/2
 >             x    ,      y, -z+1/2
 >           -y    ,  x-y, -z+1/2
 > 
 >            I would have expected these operators to produce the six possible
 >            orientations of a single water molecule at a given lattice site.
 > Instead,
 >            they seem to produce a doubly redundant set in which one OH bond is
 >            always fixed on the z axis. Am I missing something here?

There is no crystallographic requirement that the symmetry group
operators generate a non-redundant set of co-ordinates from the basis.
In general if an atom is at a site of high symmetry there will be
multiple operations generating that site.  Reduced symmetry lists for
special positions are listed in the International Tables for
Crystallography.

Keith Refson
-- 
Dr Keith Refson,        "Paradigm is a word too often used by those who would
Dept of Earth Sciences      like to have a new idea but cannot think of one." 
Parks Road,                  -- Mervyn King, Deputy Governor, Bank of England
Oxford OX1 3PR, UK        
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             earth.ox.ac.uk         Fax: 01865 272072