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Definition of triclinic crystall structure
#10285 04/18/06 06:54 PM
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leo Offline OP
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Dear All,

I have a question about the definition of a triclinic crystal structure. The following is my definition:

calc a = 29.97
calc b = 29.97
calc c = 50.00

crystal define TRIC a b c 90.0 90.0 60.0

If my understanding is correct, z direction in Cartesian coordinate is as same as c direction. however, I am confused about the relation between a or b with x or y in Cartesian coordinates. Because it will affect the value for gamma, for example, in the following case:


------
/___/


gamma could be either 60 degree or 120 degree.

Could you help to understand this? Thanks in advance.

-Leo

Re: Definition of triclinic crystall structure
leo #10286 04/18/06 07:53 PM
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Since two of the angles are 90 deg, the lattice type could be monoclinic and not triclinic.

If I recall this correctly, the convention is that alpha is the angle between b+c, beta is the angle between a+c, and gamma is tha angle between a+b. I believe the obtuse angle is used (the one > 90 deg).

I think that a=X and b=Y for orthonormal shapes, but of course that's not exactly the case for monoclinic and triclinic cases.

My personal opinion is that unless your simulating a known crystal with known unit cell data, you may be asking for trouble by using a triclinic lattice.


Rick Venable
computational chemist

Re: Definition of triclinic crystall structure
leo #10287 04/18/06 08:49 PM
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Thanks, Rick.

I do appreciate your detailed explanation and advice.

-Leo

Re: Definition of triclinic crystall structure
leo #10288 04/19/06 09:04 PM
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I check my case again. If possible, I still need use triclinic lattice. Does anyone know what we can do to specify how the lattice is aligned with respect to the global Cartesian axes in triclinic case?

Thanks a lot,
Leo

Re: Definition of triclinic crystall structure
leo #10289 04/19/06 09:23 PM
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I believe that is determined by crystallographic convention, and not something that can be specified by the user.


Rick Venable
computational chemist

Re: Definition of triclinic crystall structure
rmv #10290 04/19/06 09:52 PM
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leo Offline OP
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I looked at my image file. It seems to me that Z=c, but the relation between x or y and a or b is highly depended on the lattice structure in xy plane.

If I rotate my space groups in xy plane with some angle, the structure can be correctly repeated which can be seen in image file printed. Otherwise, all although it can be repeated, the direction CHARMM using is almost always different from mine, which will generate a wrong image of the unit cell.


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