Dissipative Particle Dynamics. Molecular Dynamics, why slow? MD solves Newton’s equations of motion for atoms/molecules: Why MD is slow?

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Presentation transcript:

Dissipative Particle Dynamics

Molecular Dynamics, why slow? MD solves Newton’s equations of motion for atoms/molecules: Why MD is slow?

Length, Time, & Energy Scales Look at typical scales of carbon-based molecules: length 1 Å, mass 12 amu, spring constant k = 20 eV/Å 2 with a Hamiltonian What is the associated equation of motion and time scale?

Hamilton’s Equations From We get dp/dt = - k x, dx/dt = p/m Or The solution is a harmonic oscillation x(t)=Acos[ (2  t+δ)/T ] with time period

Time Scale We use the units conversion factors: 1 eV = 1.6 x joule 1 amu = 1.66 x kg 1 Å = meter 1 milli sec = sec 1 μs = sec 1 nano sec = sec 1 pico sec = sec 1 femto sec = sec

How to increase the time scale of MD? Increase mass m, instead of simulating a single atom, we simulate a lump of them. Decrease the interaction strength k, we make the interaction softer.

Lennard-Jones vs Soft Pair Potential LJ Linear soft repulsion r V(r)

Brownian/Langevin Dynamics -  p is a dissipative (frictional) force, R(t) is random force with zero average and δ-function correlated in time (white noise). Bold face for vectors.

How to solve an equation with random forces? Where  is independent Gaussian random variable with zero mean and variance 2  mk B T h. (Why?)

Statistical Ensembles Micro-canonical ensemble: fixed particle number, volume, and energy (N, V, E) Canonical ensemble: fixed particle number, volume, and temperature (N, V, T). Langevin dynamics implements a canonical ensemble.

Dissipative Particle Method The dissipative particle dynamics was first proposed by Hoogerbrugge and Koelman (1992) for simulating hydrodynamic behavior. It was further improved by Groot and Warren (1997). It is a molecular dynamics with pair forces of three types.

DPD equations The forces are pair-wise additive, with conservative force F C, dissipative force F D, and stochastic force F R.

Conservative Force V(r) = (1/2) a (r –r c ) 2, r < r c = 0, r ≥ r c Where r is distance between two given particles. What is the form of the force acting on particle i from particle j ?

Dissipative Force and Random Force

Velocity-Verlet Algorithm What value to take? Order of accuracy?

Application of DPD method Coarse-grained description for solutions (e.g., water), simulating polymers (e.g., DNAs) in solution. Complex fluids at hydrodynamic time scales. Suspension of hard objects (spheres, rods, etc) in fluids.

Smoothed Particle Hydrodynamics A typical class of methods where continuum field equations (such as hydrodynamic equations) are simulated using the concept of particles. The traditional methods was to solve the partial differential equations on a regular grids (in space and time).