Effective dynamics for the (overdamped) Langevin equation

Transcription

1 Effective dynamics for the (overdamped) Langevin equation Frédéric Legoll ENPC and INRIA joint work with T. Lelièvre (ENPC and INRIA) Enumath conference, MS Numerical methods for molecular dynamics EnuMath conference, Leicester, Sept 5-9, 2011 p. 1

2 Molecular simulation Some quantities of interest in molecular dynamics: thermodynamical averages wrt Gibbs measure: Φ = Φ(X) dµ, dµ = Z 1 exp( βv (X)) dx, X R n R n or dynamical quantities: diffusion coefficients rate constants residence times in metastable basins EnuMath conference, Leicester, Sept 5-9, 2011 p. 2

3 Molecular simulation Some quantities of interest in molecular dynamics: thermodynamical averages wrt Gibbs measure: Φ = Φ(X) dµ, dµ = Z 1 exp( βv (X)) dx, X R n R n or dynamical quantities: diffusion coefficients rate constants residence times in metastable basins In practice, quantities of interest often depend on a few variables. Reduced description of the system, that still includes some dynamical information? EnuMath conference, Leicester, Sept 5-9, 2011 p. 2

4 Example of a biological system Courtesy of Chris Chipot, CNRS Nancy EnuMath conference, Leicester, Sept 5-9, 2011 p. 3

5 Reference dynamics We are interested in dynamical properties. Two possible choices for the reference dynamics of the system: overdamped Langevin equation: dx t = V (X t ) dt + 2β 1 dw t, X t R n Langevin equation (with masses set to 1): dx t = P t dt, X t R n, dp t = V (X t ) dt γp t dt + 2γβ 1 dw t, P t R n. For both dynamics, 1 T T 0 Φ(X t ) dt R n Φ(X) dµ, dµ = Z 1 exp( βv (X)) dx. We will mostly argue with overdamped Langevin, and next turn to Langevin. EnuMath conference, Leicester, Sept 5-9, 2011 p. 4

6 Metastability and reaction coordinate dx t = V (X t ) dt + 2β 1 dw t, X t position of all atoms in practice, the dynamics is metastable: the system stays a long time in a well of V before jumping to another well: we assume that wells are fully described through a well-chosen reaction coordinate ξ : R n R ξ(x) may e.g. be a particular angle in the molecule. Quantity of interest: path t ξ(x t ). EnuMath conference, Leicester, Sept 5-9, 2011 p. 5

7 Our aim dx t = V (X t ) dt + 2β 1 dw t in R n Given a reaction coordinate ξ : R n R, propose a dynamics z t that approximates ξ(x t ). preservation of equilibrium properties: when X dµ, then ξ(x) is distributed according to exp( βa(z)) dz, where A is the free energy. The dynamics z t should be ergodic wrt exp( βa(z)) dz. recover in z t some dynamical information included in ξ(x t ). Related approaches: Mori-Zwanzig formalism, asymptotic expansion of the generator (Papanicolaou,... ), averaging principle for SDE (Pavliotis and Stuart, Hartmann,... ), effective dynamics using Markov state models (Schuette and Sarich, Lu),... EnuMath conference, Leicester, Sept 5-9, 2011 p. 6

8 A super-simple case: ξ(x,y) = x Consider the dynamics in two dimensions: X = (x, y) R 2, dx t = x V (x t, y t ) dt + 2β 1 dwt x, dy t = y V (x t, y t ) dt + 2β 1 dw y t, and assume that ξ(x, y) = x. EnuMath conference, Leicester, Sept 5-9, 2011 p. 7

9 A super-simple case: ξ(x,y) = x Consider the dynamics in two dimensions: X = (x, y) R 2, dx t = x V (x t, y t ) dt + 2β 1 dwt x, dy t = y V (x t, y t ) dt + 2β 1 dw y t, and assume that ξ(x, y) = x. Let ψ(t, X) be the density of X at time t: for any B R 2, P(X t B) = ψ(t, X)dX Introduce the mean of the drift over all configurations satisfying ξ(x) = z: x V (z, y) ψ(t, z, y) dy b(t, z) := R = E [ x V (X) ξ(x t ) = z] ψ(t, z, y) dy R and consider dz t = b(t, z t ) dt + 2β 1 db t Then, for any t, the law of z t is equal to the law of x t (Gyongy 1986) B EnuMath conference, Leicester, Sept 5-9, 2011 p. 7

10 Making the approach practical b(t, z) = R x V (z, y) ψ(t, z, y) dy = E [ x V (X) ξ(x t ) = z] b(t, z) is extremely difficult to compute... Need for approximation: EnuMath conference, Leicester, Sept 5-9, 2011 p. 8

11 Making the approach practical b(t, z) = R x V (z, y) ψ(t, z, y) dy = E [ x V (X) ξ(x t ) = z] b(t, z) is extremely difficult to compute... Need for approximation: b(z) := x V (z, y) ψ (z, y) dy = E µ [ x V (X) ξ(x) = z] R with ψ (x, y) = Z 1 exp( βv (x, y)). Effective dynamics: dz t = b(z t ) dt + 2β 1 db t Idea: b(t, x) b(x) if the equilibrium in each manifold Σ x = {(x, y), y R} is quickly reached: x t is much slower than y t. EnuMath conference, Leicester, Sept 5-9, 2011 p. 8

12 The general case: X R n and arbitrary ξ dx t = V (X t ) dt + 2β 1 dw t, ξ : R n R From the dynamics on X t, we obtain (chain rule) d [ξ(x t )] = ( V ξ + β 1 ξ ) (X t ) dt + 2β 1 ξ (X t ) db t where B t is a 1D brownian motion. EnuMath conference, Leicester, Sept 5-9, 2011 p. 9

13 The general case: X R n and arbitrary ξ dx t = V (X t ) dt + 2β 1 dw t, ξ : R n R From the dynamics on X t, we obtain (chain rule) d [ξ(x t )] = ( V ξ + β 1 ξ ) (X t ) dt + 2β 1 ξ (X t ) db t where B t is a 1D brownian motion. Introduce the average of the drift and diffusion terms: ( V b(z) := ξ + β 1 ξ ) (X) ψ (X) δ ξ(x) z dx σ 2 (z) := ξ(x) 2 ψ (X) δ ξ(x) z dx Eff. dyn.: dz t = b(z t ) dt + 2β 1 σ(z t ) db t The approximation makes sense if, in the manifold Σ z = {X R n, ξ(x) = z}, X t quickly reaches equilibrium. ξ(x t ) much slower than evolution of X t in Σ z. EnuMath conference, Leicester, Sept 5-9, 2011 p. 9

14 Some remarks Effective dynamics: dz t = b(z t ) dt + 2β 1 σ(z t ) db t OK from the statistical viewpoint: the dynamics is ergodic wrt exp( βa(z))dz. Using different arguments, this dynamics has been obtained by [E and Vanden-Eijnden 2004], and [Maragliano, Fischer, Vanden-Eijnden and Ciccotti, 2006]. In the following, we will numerically assess its accuracy derive error bounds In practice, we pre-compute b(z) and σ(z) for values on a grid (remember z is scalar), and next linearly interpolate between these values. EnuMath conference, Leicester, Sept 5-9, 2011 p. 10

15 Dimer in solution: comparison of residence times solvent-solvent, solvent-monomer: truncated LJ on r = x i x j : ( ) σ 12 σ6 V WCA (r) = 4ε 2 if r σ, 0 otherwise (repulsive potential) r12 r 6 monomer-monomer: double well on r = x 1 x 2 Reaction coordinate: the distance between the two monomers EnuMath conference, Leicester, Sept 5-9, 2011 p. 11

16 Dimer in solution: comparison of residence times solvent-solvent, solvent-monomer: truncated LJ on r = x i x j : ( ) σ 12 σ6 V WCA (r) = 4ε 2 if r σ, 0 otherwise (repulsive potential) r12 r 6 monomer-monomer: double well on r = x 1 x 2 Reaction coordinate: the distance between the two monomers β Reference dyn. Effective dyn ± ± ± ± 0.04 EnuMath conference, Leicester, Sept 5-9, 2011 p. 11

17 Accuracy assessment: some background materials EnuMath conference, Leicester, Sept 5-9, 2011 p. 12

18 Accuracy assessment: some background materials dx t = V (X t ) dt + 2β 1 dw t Let ψ(t, X) be the probability distribution function of X t : P (X t B) = ψ(t, X) dx B Under mild assumptions, ψ(t, X) converges to ψ (X) = Z 1 exp( βv (X)) exponentially fast: ψ(t, ) H (ψ(t, ) ψ ) := ψ(t, ) ln C exp( 2ρt) ψ Relative entropy is interesting because ψ(t, ) ψ 2 L 1 2H (ψ(t, ) ψ ). The larger ρ is, the faster the convergence to equilibrium. Remark: ρ is the Logarithmic Sobolev inequality constant of ψ. EnuMath conference, Leicester, Sept 5-9, 2011 p. 12

19 A convergence result dx t = V (X t ) dt + 2β 1 dw t, consider ξ(x t ) Let ψ exact (t, z) be the probability distribution function of ξ(x t ): P (ξ(x t ) I) = ψ exact (t, z) dz I On the other hand, we have introduced the effective dynamics dz t = b(z t ) dt + 2β 1 σ(z t ) db t Let φ eff (t, z) be the probability distribution function of z t. Introduce the error E(t) := R ψ exact (t, ) ln ψ exact(t, ) φ eff (t, ) We would like ψ exact φ eff, e.g. E small... EnuMath conference, Leicester, Sept 5-9, 2011 p. 13

20 Decoupling assumptions Σ z = {X R n, ξ(x) = z}, dµ z exp( βv (X)) δ ξ(x) z assume that the Gibbs measure restricted to Σ z satisfy a Logarithmic Sobolev inequality with a large constant ρ, uniform in z (no metastability in Σ z ). EnuMath conference, Leicester, Sept 5-9, 2011 p. 14

21 Decoupling assumptions Σ z = {X R n, ξ(x) = z}, dµ z exp( βv (X)) δ ξ(x) z assume that the Gibbs measure restricted to Σ z satisfy a Logarithmic Sobolev inequality with a large constant ρ, uniform in z (no metastability in Σ z ). assume that the coupling between the dynamics of ξ(x t ) and the dynamics in Σ z is weak: If ξ(x, y) = x, we request xy V to be small. In the general case, recall that the free energy derivative reads A (z) = We assume that max Σz F κ. Σ z F(X)dµ z EnuMath conference, Leicester, Sept 5-9, 2011 p. 14

22 Decoupling assumptions Σ z = {X R n, ξ(x) = z}, dµ z exp( βv (X)) δ ξ(x) z assume that the Gibbs measure restricted to Σ z satisfy a Logarithmic Sobolev inequality with a large constant ρ, uniform in z (no metastability in Σ z ). assume that the coupling between the dynamics of ξ(x t ) and the dynamics in Σ z is weak: If ξ(x, y) = x, we request xy V to be small. In the general case, recall that the free energy derivative reads A (z) = We assume that max Σz F κ. Σ z F(X)dµ z assume that ξ is close to a constant on each Σ z, e.g. λ = max ξ 2 (X) σ 2 (ξ(x)) X σ 2 (ξ(x)) is small EnuMath conference, Leicester, Sept 5-9, 2011 p. 14

23 Error estimate E(t) = error = R ψ exact (t, ) ln ψ exact(t, ) φ eff (t, ) Under the above assumptions, for all t 0, E(t) C(ξ, Initial Cond.) (λ + β2 κ 2 ) Hence, if the coarse variable ξ is such that ρ is large (fast ergodicity in Σ z ), κ is small (small coupling between dynamics in Σ z and on z t ), λ is small ( ξ is close to a constant on each Σ z ), then the effective dynamics is accurate: ρ 2 at any time, law of ξ(x t ) law of z t. Remark: this is not an asymptotic result, and this holds for any ξ. This bound may be helpful, in some cases, to discriminate between various reaction coordinates. EnuMath conference, Leicester, Sept 5-9, 2011 p. 15

24 Rough estimation in a particular case Standard expression in MD: V ε (X) = V 0 (X) + 1 ε q2 (X) : q fast direction E(t) = error = ψ exact (t, ) ln ψ exact(t, ) φ eff (t, ) R If ξ q = 0, then the direction ξ is decoupled from the fast direction q, hence ξ is indeed a slow variable, and it turns out that E(t) = O(ε). If ξ q 0, then the variable ξ does not contain all the slow motion, and bad scale separation: E(t) = O(1), hence the laws of ξ(x t ) and of z t are not close one to each other. The condition ξ q = 0 seems important to obtain good accuracy. EnuMath conference, Leicester, Sept 5-9, 2011 p. 16

25 Tri-atomic molecule B B A C A C V (X) = 1 2 k 2 (r AB l eq ) k 2 (r BC l eq ) 2 + k 3 W DW (θ ABC ), k 2 k 3, where W DW is a double well potential. Reaction coordinate Orthogonality Reference Residence time condition residence time using Eff. Dyn. ξ 1 (X) = θ ABC yes ± ± ξ 2 (X) = rac 2 no ± ± EnuMath conference, Leicester, Sept 5-9, 2011 p. 17

26 Extension to the Langevin equation We have considered until now the overdamped Langevin equation: dx t = V (X t ) dt + 2β 1 dw t Consider now the Langevin equation: dx t = P t dt dp t = V (X t ) dt γp t dt + 2γβ 1 dw t Can we extend the numerical strategy? Joint work with F. Galante and T. Lelièvre. EnuMath conference, Leicester, Sept 5-9, 2011 p. 18

27 Building the effective dynamics - 1 We compute dx t = P t dt, dp t = V (X t ) dt γp t dt + 2γβ 1 dw t d [ξ(x t )] = ξ(x t ) P t dt We introduce the coarse-grained velocity and have (chain rule) v(x, P) = ξ(x) P R d [v(x t, P t )] = [ Pt T 2 ξ(x t )P t ξ(x t ) T V (X t ) ] dt γv(x t, P t ) dt + 2γβ 1 ξ(x t ) db t where B t is a 1D Brownian motion. We wish to write a closed equation on ξ t = ξ(x t ) and v t = v(x t, P t ). Introduce D(X, P) = P T 2 ξ(x)p ξ(x) T V (X). EnuMath conference, Leicester, Sept 5-9, 2011 p. 19

28 Building the effective dynamics - 2 Without any approximation, we have obtained dξ t = v t dt, dv t = D(X t, P t ) dt γv t dt + 2γβ 1 ξ(x t ) db t To close the system, we introduce the conditional expectations with respect to the equilibrium measure µ(x, P) = Z 1 exp[ β ( V (X) + P T P/2 ) ]: Effective dynamics: D eff (ξ 0, v 0 ) = E µ ( D(X, P) ξ(x) = ξ0, v(x, P) = v 0 ) σ 2 (ξ 0, v 0 ) = E µ ( ξ 2 (X) ξ(x) = ξ 0, v(x, P) = v 0 ) dξ t = v t dt, dv t = D eff (ξ t, v t ) dt γv t dt + 2γβ 1 σ(ξ t, v t ) db t Again, this dynamics is consistent with the equilibrium properties: it preserves the equilibrium measure exp[ βa(ξ 0, v 0 )] dξ 0 dv 0. EnuMath conference, Leicester, Sept 5-9, 2011 p. 20

29 Numerical results: the tri-atomic molecule B B A C A C V (X) = 1 2 k 2 (r AB l eq ) k 2 (r BC l eq ) 2 + k 3 W DW (θ ABC ), k 2 k 3, where W DW is a double well potential. Reaction coordinate: ξ(x) = θ ABC. Inverse temp. Reference Eff. dyn. β = ± ± β = ± ± 1.25 Excellent agreement on the residence times in the well. EnuMath conference, Leicester, Sept 5-9, 2011 p. 21

30 Pathwise accuracy (ongoing work with T. Lelièvre and S. Olla) Going back to the overdamped case, with ξ(x, y) = x, can we get pathwise accuracy, e.g. [ ] E x t z t 2 C(T)? ρ sup 0 t T EnuMath conference, Leicester, Sept 5-9, 2011 p. 22

31 Pathwise accuracy (ongoing work with T. Lelièvre and S. Olla) Going back to the overdamped case, with ξ(x, y) = x, can we get pathwise accuracy, e.g. [ ] E x t z t 2 C(T)? ρ sup 0 t T z t is the effective dynamics trajectory: dz t = b(z t ) dt + 2β 1 db t (x t, y t ) is the exact trajectory: dx t = x V (x t, y t ) dt + 2β 1 db t = b(x t ) dt + e(x t, y t ) dt + 2β 1 db t By construction, b(x) = E µ [ x V (X) ξ(x) = x], hence x, E µ [e(x) ξ(x) = x] = 0. EnuMath conference, Leicester, Sept 5-9, 2011 p. 22

32 Poisson equation Let L x be the Fokker-Planck operator corresponding to a simple diffusion in y, at fixed x: For any x, L x φ = div y (φ y V ) + β 1 y φ E µ [e(x) ξ(x) = x] = 0 = u(x, ) s.t. (L x ) u(x, ) = e(x, ) Assume that L x satisfies a (large) spectral gap ρ 1. Then, u C/ρ 1. We have not been able to show a bound on E [ sup 0 t T x t z t 2], but we have shown that ( P sup 0 t T x t z t cρ α ) C ln(ρ) for any 0 α < 1/2. This somewhat explains the good results we observe on the residence times. EnuMath conference, Leicester, Sept 5-9, 2011 p. 23

33 Numerical illustration: the tri-atomic molecule B A C V (X) = 1 2 k 2 (r AB l eq ) k 2 (r BC l eq ) 2 + k 3 W(θ ABC ), k 2 k θ ABC (X t ) z t t EnuMath conference, Leicester, Sept 5-9, 2011 p. 24

34 Conclusions We have proposed a natural way to obtain a closed equation on ξ(x t ). Encouraging numerical results and rigorous error bounds (marginals at time t and in probability on the paths). Once a reaction coordinate ξ has been chosen, computing the drift and diffusion functions b(z) and σ(z) is as easy/difficult as computing the free energy derivative A (z). The approach can be extended to the Langevin equation. FL, T. Lelièvre, Nonlinearity 23, FL, T. Lelièvre, Springer LN Comput. Sci. Eng., vol. 82, 2011, arxiv FL, T. Lelièvre, S. Olla, in preparation F. Galante, FL, T. Lelièvre, in preparation EnuMath conference, Leicester, Sept 5-9, 2011 p. 25