Multi-scale Modeling using the Adaptive Resolution Scheme

Transcription

1 Multi-scale Modeling using the Adaptive Resolution Scheme Christoph Junghans Los Alamos National Laboratory NM, USA Oct. 9, 2012 UNCLASSIFED(LA-UR ) 1

2 Coworkers A lot of people have contributed to AdResS: Cecilia Clementi (Rice University, Texas) Luigi Delle Site (MPI-P FU Berlin) Sebastian Fritsch (MPI-P) Kurt Kremer (MPI-P) Brad Lambeth (Rice University, Texas) Debashish Mukherji (MPI-P) Patrick Kiley (University of Cambridge) Simón Poblete (MPI-P FZ Jülich) Adolfo Poma (MPI-P Sapienza University of Rome) Matej Praprotnik (MPI-P National Institute of Chemistry, Slovenia) Stac Bevc (National Institute of Chemistry, Slovenia) UNCLASSIFED(LA-UR ) 2

3 Introduction The Adaptive Resolution Scheme (AdResS) The big idea 1 Coupling of two resolutions (AA and CG) by a force interpolation: F αβ = w α w β F AA αβ +[1 w αw β ] F CG αβ, where α and β are molecules 1 M. Praprotnik, L. Delle Site, and K. Kremer, JCP 123, (2005). UNCLASSIFED(LA-UR ) 3

4 Introduction Illustration Magnifier: Free motion: UNCLASSIFED(LA-UR ) 4

5 Introduction Motivation Inherited from coarse-graining Accessibility of bigger time- and length-scales Avoids sampling of unimportant degrees of freedom Better sampling through smoother energy landscape Computational speed-up As an analysis tool Study influence of coarse-graining Replace surrounding environment UNCLASSIFED(LA-UR ) 5

6 Introduction What kind of systems can be coupled? Atomistic and coarse-grained representation of the same system Atomistic and very simplified (LJ) system Quantum and atomistic systems Coupling conditions 2 ρ at = ρ cg, T at = T cg, (p at = p cg ) 2 M. Praprotnik, L. Delle Site and K. Kremer; Annu. Rev. Phys. Chem. 59, 545 (2008). UNCLASSIFED(LA-UR ) 6

7 Methodic Details The Weighting Function I 1 Atomistic Hybrid Coarse-grained w(x) 0 a 1 : atomistic/explicit region w(x) = 0 < w < 1 : hybrid region 0 : coarse-grained region b x UNCLASSIFED(LA-UR ) 7

8 Methodic Details The Weighting Function II 1 Atomistic Hybrid Coarse-grained w(x) 0 a b x Boundary conditions Monotonic No artificial barriers w (d ex ) = 0 = w (d ex +d hy ) smooth free energy transition UNCLASSIFED(LA-UR ) 8

9 Methodic Details The Weighting Function III 1 Atomistic Hybrid Coarse-grained w(x) 0 a ( 0 ) : x> d ex +d hy w(x) = cos 2 π 2d hy (x d ex ) : d ex +d hy >x> d ex 1 : d ex >x UNCLASSIFED(LA-UR ) 9 b x

10 AdResS at a First Glance Radial Distribution Function (Tetrahedral Liquid) g(r) [1] Explicit simulation AdResS r [σ] AdResS as a magnifier for a certain region RDF matches and accuracy depends more on the accuracy of the coarse-graining than on AdResS UNCLASSIFED(LA-UR ) 10

11 AdResS at a First Glance Particle Fluctuations t = 0 t = 100 t = t = 0 t = 100 t = N N CG HY AT 10 CG HY AT x x Particles can freely diffuse through the hybrid region Profile is slightly asymmetric Coarse-grained dynamics is slightly faster UNCLASSIFED(LA-UR ) 11

12 AdResS at a First Glance Density Artifacts in the Hybrid Region There is no free lunch! ρ/ρ0 [1] CG 10 HY 20 x [σ] EX 30 System is inhomogeneous in the hybrid region Linear interpolation is too simple Can be corrected, but is not necessary in all cases UNCLASSIFED(LA-UR ) 12

13 Theory of the Thermodynamic Force Idea Chemical potential in the hybrid region is different Add an external field 3 fthermo = (µ AA µ(w)) µex w to the linear interpolation to repair the density inhomogeneity Correction is not always necessary Allows to couple systems of different pressures 3 S. Poblete, M. Praprotnik, K. Kremer and L. Delle Site, JCP 132, (2010) UNCLASSIFED(LA-UR ) 13

14 Theory of the Thermodynamic Force Pressure View Chemical potential is tedious to measure Assumption of constant weight To obtain a flat density profile one needs to compensate for the different pressures in the hybrid region 4 fthermo = 1 ρ 0 (p AA p(w)) Local pressure depends on the local density fthermo = 1 ρ 2 0 κ (ρ(w)) Easier, because ρ(w) can be measured on the fly Iterative correction is possible 4 S. Poblete, S. Fritsch, C. Junghans, G. Ciccotti, K. Kremer and L. Delle Site, PRL 108, (2012) UNCLASSIFED(LA-UR ) 14

15 Numerical Details Overview I The problem is the hybrid region! Question How to reintroduce the atomistic details? How to initialize the atomistic velocities? How to introduced the correct bond distribution? How to map the molecular force interpolation to the atoms in the atomistic region? UNCLASSIFED(LA-UR ) 15

16 Numerical Details Overview II Consumed Computer Times in MD 70% Force calculation 15% Communication 8% Neighbor search 5% Position update 2% IO Position update is cheap do NOT remove atomistic particles, but do not treat them force-wise and integrate them everywhere. Double representation trick 5 ( everywhere) 5 C. Junghans and S. Poblete, Comp. Phys. Comm. 181, 1449 (2010). UNCLASSIFED(LA-UR ) 16

17 Numerical Details Virtual Sites How to implement the double representation? Virtual sites: Dummy particles, which are moved according to a geometrical rule rather than by Newton s equation of motion. Force distribution Force on an atom in an molecule with a virtual site: F i atom = (V +Vcg ) = F r i ex + cg F cg r COM = F ex m i + i i r i i α m F cg i Can be easily added to the integrator! UNCLASSIFED(LA-UR ) 17

18 Numerical Details Integrator Velocity Verlet Integrator (with AdResS and virtual sites) 1. v(t + t/2) = v(t)+ t/2f(t)/m 2. p(t + t) = p(t)+ tv(t + t/2) 2b. Update the positions, velocities and weighting functions w( R) of the virtual sites 3. Calculate f(t + t) from p(t + t), v(t + t/2) (and thermostat) 3b. Distribute the force of the virtual sites to the real particles 4. v(t + t) = v(t + t/2)+ t/2f(t + t) Virtual sites increase the amount of communication (O(N 2/3 VS )). UNCLASSIFED(LA-UR ) 18

19 Numerical Details Thermostat A thermostat is needed for a smooth transition. Langevin Thermostat Thermostat force acting on the atoms f thermo i = f D i + f R i = ξ i v i +σ i η i. One can show that i α f thermo i = F D α + F R α is also a Langevin thermostat with a common fluctuation-dissipation theorem (σ 2 = k B Tξ) if the friction constants ξ i have been scaled with the masses, ξ i = m i ξ. DPD thermostat can also be used, but more theory is involved. UNCLASSIFED(LA-UR ) 19

20 Numerical Details Reinitialization Possiblities for Velocity and Bond Reinitialization Initialization from Maxwell/bond distribution Freeze and unfreeze them Keep on integrating them Reminder: We have hybrid molecules everywhere Bonded interactions are cheap keep on integrating them is the easiest solution UNCLASSIFED(LA-UR ) 20

21 Numerical Details Cut-Offs AdResS needs a molecule-based cut-off due to the force interpolation. Molecular Cut-Offs Avoid that molecules interact half atomistic - half coarse-grained Avoid dipoles at the cut-off UNCLASSIFED(LA-UR ) 21

22 Numerical Details Speed-up Simulation of a big system with small atomistic region: 6 Ideal strong speed-up (const. systemsize) s 1 = ( tnew t old ) V AA <<V = CG t not-forces + t forces t total Toluene = t total (atoms per molecule) S. Fritsch, C. Junghans and K. Kremer, JCTC 8, 398 (2012). UNCLASSIFED(LA-UR ) 22

23 Basic Examples Tetrahedral Liquid Toy model for a liquid (methane-like). 7 Explicit (all-atom) Interactions Non-bonded (even inside the molecules), LJ: U ex LJ (r iαjβ) = Bonded, FENE: U ex FENE (r iαjα) = Coarse-grained Interactions { 4ǫ [ (σ/r iαjβ ) 12 (σ/r iαjβ ) ], riαjβ 2 1/6 σ 0, r iαjβ > 2 1/6 σ { 1 2 kr2 0 ln[ 1 (r iαjβ /R 0 ) 2], r iαjβ R 0 r iαjβ > R 0 Isotropic one-site tabulated potential U(r ij ) by IBI UNCLASSIFED(LA-UR 7 C. Junghans ) and S. Poblete, Comp. Phys. Comm. 181, 1449 (2010). 23

24 Basic Examples Tetrahedral Liquid (Structure) g(r) [1] Explicit simulation No correction ip correction tf correction r [σ] UNCLASSIFED(LA-UR ) 24

25 Basic Examples Tetrahedral Liquid (Density profile) 1.4 CG HY EX ρ/ρ0 [1] no correction ρ/ρ0 [1] tf correction x [σ] UNCLASSIFED(LA-UR ) 25

26 Basic Examples Water A realistic, but simple system Electrostatics by Reaction Field One bead coarse-grained model Can we couple a not pressure-corrected model, too? 8 8 S. Poblete, S. Fritsch, C. Junghans, G. Ciccotti, K. Kremer and L. Delle Site, PRL 108, (2012) UNCLASSIFED(LA-UR ) 26

27 Basic Examples Water (Coarse-grained Model) U(r) [kj/mol] g(r) [1] p cg = p ex κ cg = κ at full atomistic p cg = p ex κ cg = κ at r [nm] UNCLASSIFED(LA-UR ) 27

28 Basic Examples Water (Perturbation without Thermodynamic Force) g(r) [1] r [nm] d ex = (AA) d ex = 0.20 nm d ex = 0.35 nm d ex = 0.55 nm d ex = 1.00 nm d ex = 0 (CG) UNCLASSIFED(LA-UR ) 28

29 Basic Examples Water (Density profile) ρ/ρ0 [1] uncorrected AdResS it 1 it 2 it 3 it AA Hy CG x [nm] One can even bring non-pressure corrected model in equilibrium 16 UNCLASSIFED(LA-UR ) 29

30 Basic Examples Water (Thermodynamic Force) 12 AA Hy CG 10 Ftf (x) [kj/mol/nm] x [nm] UNCLASSIFED(LA-UR ) 30

31 Basic Examples Water (Diffusion Profile) ρ/ρ0 [1] AA Hy 2ps 100ps 200ps 300ps 400ps CG x [nm] UNCLASSIFED(LA-UR ) 31

32 Basic Examples Water (Particle Fluctuations in AA zone) 0.02 AdResS All-atom Gaussian approximation p(n) Grand-canonical behaviour 6350 N UNCLASSIFED(LA-UR ) 32

33 Basic Examples Water (Particle Fluctuations) < N 2 > < N > 2 / < N > AA Hy AdResS p cg = p ex AdResS κ cg = κ at full atomistic CG x [nm] UNCLASSIFED(LA-UR ) 33

34 Basic Examples Water (Thermodynamic Force) AA Hy CG 6000 p [bar] local p xx ρ 0 /M α x a F thdx x [nm] Thermodynamic force compensates the pressure difference. 15 UNCLASSIFED(LA-UR ) 34

35 Outlook AdResS: Useful analysis tool Speed-up simulations Molecules exchange between the two resolutions Things to work on: Electrostatics Adaptive implementation ESPResSo ++ UNCLASSIFED(LA-UR ) 35

36 The End Thank you for your attention! UNCLASSIFED(LA-UR ) 36