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[ccp4bb]: stable Ih ice and P63/mmc space group
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Two related questions. One relates to simulation, the other to crystals
so I'm posting to two lists.
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.
I ended up using Argon and got nice results, but my first attempt
with H2O has me a bit puzzled.
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.
I can simply choose four unique H2O molecules per unit cell
and get a nice lattice with no clashes, which is what I did.
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.
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?
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.
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?
yes, I am applying these in fractional coordinates and shifting the
back into the unit cell, The fractional coordinates of the H2O
with a,b = 4.516 and c = 7.354
angles = (90,90,120)
Cornell Theory Center