Workshop on HPC in India

Modelling Soft MatterBalasubramanian Sundaram

Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific

Research, Bangalore

ATIP 1st Workshop on HPC in India @ SC-09

Balasubramanian Sundaram

ATIP 1st Workshop on HPC in India @ SC-09 2

Current Challenge

Vesicle interacting with a lipid membrane

Shinoda and Klein, Science (2008)

Balasubramanian Sundaram

ATIP 1st Workshop on HPC in India @ SC-09 3

Molecular dynamics

Obtain Forces,

Integrate

t

Balasubramanian Sundaram

ATIP 1st Workshop on HPC in India @ SC-09 4

Parallelization strategies Decomposition

based on particle indices (works for < 1000 particles)

Domain/Spatial decomposition (for large system sizes ~ 1 million)

Decomposition based on particle indices (works for < 1000 particles)

Domain/Spatial decomposition (for large system sizes ~ 1 million)

System of interacting particles

Balasubramanian Sundaram

ATIP 1st Workshop on HPC in India @ SC-09 5

Approaches Density functional theory based MD (ab

initio molecular dynamics) Accurate, but expensive

Empirical force field based atomistic MD Less accurate than DFT, but reasonable; not so

expensive Coarse grain MD

Softer forces, fewer degrees of freedom, ideal to study mesoscale phenomena

Balasubramanian Sundaram

ATIP 1st Workshop on HPC in India @ SC-09 6

Hierarchical modelling Perform ab initio MD (AIMD)

calculations. Obtain information on intermolecular structure

Refine empirical force-field parameters so that the results of atomistic MD match those from AIMD.

Use results of atomistic MD to carry out coarse grain simulations

Balasubramanian Sundaram

ATIP 1st Workshop on HPC in India @ SC-09 7

Challenges

Length and time scales of processes and phenomena are large (tens of nanometers, micro to milliseconds)

Atomistic description is crucial to understand the chemistry of specific substances

How does one build models starting from atomistic ones, to reach the large length/time scales?

Balasubramanian Sundaram

ATIP 1st Workshop on HPC in India @ SC-09 8

Multiscale Modeling

Atomistic Coarse GrainAb Initio

Balasubramanian Sundaram

ATIP 1st Workshop on HPC in India @ SC-09 9

Ionic compoundsNaCl

M.P.: 801 ºC

[bmim][PF6]

M.P.: 10 ºC

Low volatility Environmentally

benign Polar liquids Nanoparticle

synthesis, electrolytes, catalysis

Balasubramanian Sundaram

ATIP 1st Workshop on HPC in India @ SC-09 10

Issues

Lack of scattering data to determine intermolecular structure in the liquid state

Transferability of intermolecular potentials whose partial charges are fitted to gas phase quantum calculations

Phase behaviour Prediction of bulk properties Microheterogeneity

Balasubramanian Sundaram

ATIP 1st Workshop on HPC in India @ SC-09 11

Intermolecular structure

Anions comecloser due toelectronicpolarization

B.L. Bhargava and S. BalasubramanianJournal of Physical Chemistry B 111, 4477 (2007)

Atomistic (non-bonded): CLP model, JPCB (2004)

Balasubramanian Sundaram

ATIP 1st Workshop on HPC in India @ SC-09 12

Refined atomistic model: Predictions

Density within 1.5% of experiment between 300-500K, within 2% up to 200 MPa

Diffusion coefficients within 20% of experiment at 300K/1atm

Surface tension at 300K within 10% of experiment

Enthalpy of vaporization within 10% of experiment

B.L. Bhargava and S. Balasubramanian, J. Chem. Phys. (2007)

Refining the parameters of the atomistic model to match the intermolecular structure of AIMD provides:

Balasubramanian Sundaram

ATIP 1st Workshop on HPC in India @ SC-09 13

Intermolecular structure

B.L. Bhargava, M.L. Klein and S. Balasubramanian, ChemPhysChem (2008)

Balasubramanian Sundaram

ATIP 1st Workshop on HPC in India @ SC-09 14

Coarse graining

Example: [C7mim][PF6]

41 atoms versus 6 beads

Can be used for any [Cnmim][PF6]Studied here: n=4, 7, 10Parametrized for [C4mim][PF6]

• Around 3 heavy atoms per bead• Atom charges summed up to give bead charges• Atom dispersion strengths summed up to give bead Epsilons• Beads of alkyl tail: Model based on surfactants (Shinoda et al)

Balasubramanian Sundaram

ATIP 1st Workshop on HPC in India @ SC-09 15

Nanoscale ordering

NEUTRON X-RAY

[C4mim][PF6]

[C10mim][PF6]

[C7mim][PF6]Increasing Correlation Length

Triolo et al (2008)

SIMULATION

EXPERIMENT

BLB, RD, MLK & SB, Soft Matt. (2007)

Balasubramanian Sundaram

ATIP 1st Workshop on HPC in India @ SC-09 16

Morphology

Bicontinuous

12 nm

Anion RichTail Rich

BLB, RD, MLK & SB, Soft Matt. (2007)

Balasubramanian Sundaram

ATIP 1st Workshop on HPC in India @ SC-09 17

Symmetric tails

1-butyl,3-methylimidazoliumhexafluorophosphate

1,3-didecylimidazoliumhexafluorophosphate

Asymmetric tails

Symmetric tails

Dzyuba and BartschChemcomm (2001)

“The didecyl compoundhas two melting points:-27oC and 16oC”

Raju and Balasubramanian, J. Mater. Chem. (2009)

Balasubramanian Sundaram

ATIP 1st Workshop on HPC in India @ SC-09 18

Emergence of lamellar phase

After 50 ns After 450 ns

Sectional View:After 450 ns

Lamellar PhaseSelf Assembly

Raju and Balasubramanian, J. Mater. Chem. (2009)

Balasubramanian Sundaram

ATIP 1st Workshop on HPC in India @ SC-09 19

Summary

Development of a hierarchical approach to model room temperature ionic liquids

Ab initio, atomistic and coarse grain simulations carried out

Quantitative comparison to experiments at each stage

We predict the formation of a lamellar phase in ionic liquids containing cations with symmetric long alkyl groups.