ZMC: A Tool for Modelling Diffuse Scattering from Single Crystals

1

• ZMC A Tool for Modelling Diffuse Scattering from
  Single Crystals

D.J.Goossens AINSE Fellow Research School of
Chemistry Australian National University
2

• Whats the problem?

• Modelling Bragg data -- use unit cell
• (asymmetric unit symmetry)
• But the whole point of diffuse scattering is SRO.
• Means you cannot treat unit cells as the same
• Looking for the population of local
  configurations.
• So you need a model big enough to contain a
  statistically useful population of local
  configurations (around 32 ? 32 ? 32 unit cells).
• Too many atoms to fit their positions directly.

3

• Whats the problem?

• Too many atoms to fit their positions directly.
• So instead work with the interactions that
  determine the positions.
• Parameterise these interactions
• These parameters become the parameters of the
  model.
• In this case, we are interested in modelling the
  diffuse scattering from flexible molecular
  crystals.

4
(No Transcript)
5
(No Transcript)
6
(No Transcript)
7
Within a molecule
8
Within a molecule
dcv2.4Å
9
Within a molecule
10
Between molecules
To correlate the thermal motions, we connect the
molecules with contact vectors (cv)
11
(No Transcript)
12
(No Transcript)
13
Key points of approach
Describe molecule by z-matrixAllow it to
flip/reorientAllow another molecule to
substitute for itAllow molecule to flexPut
potentials between and within moleculesAllow for
interaction of occupancy and displacement Then
do MC Then calculate diffuse scattering Then
interrogate the model
14
Key points of approach
Describe molecule by z-matrixAllow it to
flip/reorientAllow another molecule to
substitute for itAllow molecule to flexPut
potentials between and within moleculesAllow for
interaction of occupancy and displacement Then
do MC Then calculate diffuse scattering Then
interrogate the model
15
Para-terphenyl
16
Information
17
Benzocaine
18
Information
19
Information
20
Thanks
Prof. Richard Welberry Dr Aidan Heerdegen Dr Eric
Chan Mr Andrew Beasley Prof. W.I.F David AINSE,
ARC, AMRFP, ASRP