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Phase behavior of Methane Clusters from MD Calculations . Ana Proykova University of Sofia Gordon Research Conference on Molecular & Ionic Clusters Centre Paul Langevin , Aussois, France September, 7, 2004. Interest in:.

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phase behavior of methane clusters from md calculations

Phase behavior of Methane Clusters from MD Calculations

Ana Proykova

University of Sofia

Gordon Research Conference on Molecular & Ionic Clusters

Centre Paul Langevin, Aussois, France

September, 7, 2004

interest in
Interest in:
  • What is the actual state (liquid or solid) of a nano-sized particle in a contact with a surface?
  • Charge dependence of the interaction potential

Important for Biology (penetration of gases into cells), Green-house effect, clathrates

One way to answer these questions is a simulation of clusters of various atoms and molecules, interacting with different potentials

local icosahedral ordering in the liquid phase franck
Local icosahedral ordering in the liquid phase - Franck
  • the structure of atomic liquids and glasses

has a polytetrahedral arrangement

Is it true for molecular systems?

Generally – no!

Clusters of octahedral molecules pack in solid structures below their freezing point (size dependent)

normal modes of ch4 ignored in our md
Normal modes of CH4 (ignored in our MD)
  • Stretches [A1+T2] 3215 (A1) 3104 (T2) {cm -1}
  • Bends [E + T2] 1412 (E) 1380 (T2) {cm -1}
  • T2 modes IR active
  • All modes are Raman active
  • The symmetric C–H stretching mode of the CH4 molecule in water is a single peak at 2910{cm -1}with a half-width of approximately 5 {cm -1}
  • Octopole moments of CH4 estimated from the T dependence of second virial coefficients 3.7 10-34 esu. cm 3
slide7

Molecular dynamics simulations at a constant total energy and free cluster surface: velocity Verlet algorithm for solving the classical equations of motion

  • N - rigid molecules (the lowest frequency 1380 1/cm is much higher than the fastest inter-molecular vibrations < 250 1/cm)
  • Lennard-Jones (short-range) & Coulomb (long-range) potential – q on the C-atom is negative (0.2 - 1 e)
range of potential and structure
Range of potential and structure
  • Long-range – highly strained, highly coordinated, spherical structures : no regular packing. For large size – liquid-like inherent structure
  • Intermediate ranges – icosahedral structures are dominant
  • Short – range – decahedral
  • Very short – fcc structure dominates
phase diagram
Phase Diagram
  • High temperature phase: molecular axes are random, rotational and translational diffusion
  • Low temperature phase: rotational (librational) diffusion
  • ΔT = |Ttr.p – Tcr.p |→ 0 for N≤50
numerical diagnostics
Numerical diagnostics

Radial distribution function

Lindemann criterion

Normal modes analysis – diagonalization of Hessian

slide16
The lowest energy starting configuration (no kinetic energy) of a 55-molecule cluster is the icosahedron
slide17
Radial distribution functions (RDF) for selected structures: ‘real’ indicates the RDF for a 55 molecule cluster at 10 K
slide18

10 K

The most important pattern is the double peak at 4 A:the nearest neighbors form 'anti-aligned 1' dimers
what new
What new?
  • The most important finding in this study is that dimers of molecules with a specific mutual anti-ferro orientation play an important role in cluster ordering at low temperatures.

This result – not expected for rigid, globular-like molecules – is probably due to the angular dependence of the interaction.

slide24
Team
  • Dr. R. Radev (currently, financial company)
  • Mr. S. Pisov(U of Sofia – Ass. Prof.)
  • Ms. E. Daykova(U of Sofia – PhD student)
  • Mr. H. Iliev(U of Sofia – PhD student)
acknowledgements
Acknowledgements
  • NFS – Bulgaria
  • U of Sofia – Scientific Grants
  • European Commission grants for mobility
  • Resources (EPCC, TRACS)
  • Discussions with R.S. Berry (U of Chicago)
thank you for listening
Thank you for listening

and the Chairs of this meeting

for selecting the topic