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*Subject*: [ccp4bb]: stable Ih ice and P63/mmc space group*From*: Richard Gillilan <richard@tc.cornell.edu>*Date*: Tue, 23 May 2000 12:19:22 -0400*CC*: chemistry@ccl.net, ccp4bb@dl.ac.uk*References*: <001001bfc142$515cc5e0$0200a8c0@pavilion>*Sender*: owner-ccp4bb@dl.ac.uk

*** For details on how to be removed from this list visit the *** *** CCP4 home page http://www.dl.ac.uk/CCP/CCP4/main.html *** 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. 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? yes, I am applying these in fractional coordinates and shifting the result back into the unit cell, The fractional coordinates of the H2O molecule are: O (.333,.666,.063) H (.333,.666,.193) H (.455,.910,.018) with a,b = 4.516 and c = 7.354 angles = (90,90,120) Thanks Richard Gillilan Cornell Theory Center

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