High-order Chin actions in path integral Monte Carlo

High-order Chin actions in path integral Monte Carlo High-order actions and their applications, Barcelona 2009 Jordi Boronat Departament de Física i Enginyeria Nuclear Universitat Politècnica de Catalunya Barcelona, Spain In collaboration with: Joaquim Casulleras, Konstantinos Sakkos and Riccardo Rota High-order actions 09 p.1/28

Quantum many-body theory at T > 0 At T = 0, several options: perturbative series, variational method, integral equations (HNC),... Also Monte Carlo: VMC; GFMC and DMC = Exact results for bosons and probably the best ones for fermions For T > 0, the problem becomes more difficult and the number of possible approaches reduces Monte Carlo + Path Integral (Feynman) (PIMC) has proven to be one of the best options... if not the only reliable one for correlated systems High-order actions 09 p.2/28

Density matrix in Statistical Mechanics Thermal density matrix: ˆρ = e βĥ, with Ĥ the Hamiltonian of the system and β = 1/T The expectation value of any operator O is O = Z 1 i φ i O φ i e βe i with Z = i e βe i the partition function Projecting to the coordinate space, O = Z 1 drdr ρ(r,r ;β) R O R with ρ(r,r ;β) = i e βe i φ i (R)φ i (R ) High-order actions 09 p.3/28

Convolution property of the density matrix The density matrix can always be decomposed as ρ(r 1,R 2 ;β) = dr 3 ρ(r 1,R 3 ;β/2)ρ(r 3,R 2 ;β/2) Important: We get information at a temperature T = 1/β from knowledge at a temperature twice larger T = 2/β. By iterating M times, ρ(r 0,R M ;β) = dr 1... dr M 1 ρ(r 0,R 1 ;ǫ)... ρ(r M 1,R M ;ǫ) with ǫ = β/m High-order actions 09 p.4/28

Trotter formula Exact result for ρ(r,r ;β) would require to know the full spectrum of H: impossible in practice High-order actions 09 p.5/28

Trotter formula Exact result for ρ(r,r ;β) would require to know the full spectrum of H: impossible in practice Consider Ĥ = ˆK + ˆV. Using the Baker-Campbell-Hausdorff formula, e ǫ ˆKe ǫˆv = e ǫ( ˆK+ˆV ) e ǫ2 C 2 ǫ 3 C 3 +... with C 2 = 1 2 [ ˆK, ˆV ] and C 3 = 1 12 [ ˆK ˆV, [ ˆK, ˆV ]] High-order actions 09 p.5/28

Primitive Approximation In a first approximation (Primitive Approximation (PA)), terms of order ǫ 2 and higher are neglected e ǫ( ˆK+ˆV ) ǫ = e ˆKe ǫˆv Kinetic and potential terms are easily evaluated R e ǫ( ˆK+ˆV ) R = dr R e ǫ ˆK R R e ǫˆv R since they can be computed separately High-order actions 09 p.6/28

Primitive Approximation The partition function is (R {r 1,...,r N }) Z = dr 1... dr M M α=1 ρ PA (R α,r α+1 ) with R M+1 = R 1 Introducing explicitly the kinetic and potential terms ρ PA (R α,r α+1 ) = ( ) 3N/2 Mm 2πβ 2 exp N i=1 Mm 2β 2 (r α,i r α+1,i ) 2 β M N V (r α,ij ) i<j High-order actions 09 p.6/28

Mapping the quantum problem to a classical one The quantum problem can be mapped to a classical problem of polymers (Chandler & Wolynes (1981)) Every quantum particle is described as a polymer with a number of beads which increases when the temperature T decreases Every bead interacts with all the beads having the same index through V (r); harmonic coupling between successive beads of a given particle exp [ Mm ] 2β 2(r α,i r α+1,i ) 2 High-order actions 09 p.7/28

Mapping the quantum problem to a classical one 1 0.5 0-0.5-1 -1.5-2 -2.5-3 -2-1.5-1 -0.5 0 0.5 1 1.5 2 1.5 1 0.5 0-0.5-1 -1.5-2 -2.5-2 -1.5-1 -0.5 0 0.5 1 1.5 2 5 H 2 molecules 5 H 2 molecules with 32 beads at T = 6 K with 256 beads at T = 1 K High-order actions 09 p.7/28

Convergence of PA The primitive approximation is accurate to second order in ǫ 2 0.51 0.505 1D Harmonic oscillator at T = 0.2 E 0.5 0.495 0.49 0.485 0.48 0 0.02 0.04 0.06 0.08 0.1 0.12 1/M Reasonable accuracy for semiclassical problems Not enough for quantum liquids, especially for their superfluid phases; in liquid 4 He ( 3000 beads = slowing down) High-order actions 09 p.8/28

First correction to PA: Takahashi- Imada Takahashi & Imada (1984), and independently Li & Broughton (1987), proposed a new action with a trace accurate to order ǫ 4 The double commutator [[ˆV, ˆK], ˆV ] = 2 /m( V ) 2 is introduced, and the bare potential ˆV = i<j V (r α,ij) is substituted by Ŵ TI = N i<j V (r ij ) + 1 24 ( β M ) 2 Ŵ with F i = N j i iv (r ij ) and Ŵ = ( 2 /m) N i=1 F i 2 High-order actions 09 p.9/28

First correction to PA: Takahashi- Imada 0.52 156 0.50 0.48 160 E 0.46 E/N 164 0.44 0.42 0.00 0.05 0.10 0.15 0.20 0.25 1/M 1D Harmonic oscillator T = 0.2 168 0.0 0.1 0.2 0.3 0.4 0.5 1/M Liquid Ne, ρ = 0.0363 Å 3 T = 25.8 K L. Brualla, K. Sakkos, J. B., and J. Casulleras, J. Chem. Phys. 121, 636 (2004) High-order actions 09 p.9/28

Possible paths for improvement Takahashi-Imada (TIA) behaves as ǫ 4 ; not enough for reaching efficiently superfluid regimes High-order actions 09 p.10/28

Possible paths for improvement Takahashi-Imada (TIA) behaves as ǫ 4 ; not enough for reaching efficiently superfluid regimes Ceperley & Pollock introduced the pair action (PDM) ρ(r,r ; ǫ) = N ρ(r i,r i; ǫ) i=1 N exp [ U(r ij,r ij; ǫ) ] i<j PDM is accurate, but not easy to use and restricted in practice to radial potentials High-order actions 09 p.10/28

Chin Action (t 0, a 1 ) (I) We chose the (t 0, a 1 ) expansion due to its higher flexibility (S. A. Chin and C. R. Chen, J. Chem. Phys. 117, 1409 (2002)); exact ǫ 6 order for the harmonic oscillator e ǫĥ e v 1ǫŴa 1e t 1 ǫ ˆTe v 2ǫŴ1 2a 1e t 1 ǫ ˆTe v 1ǫŴa 1e 2t 0 ǫ ˆT with Ŵ a1 = ˆV + (u 0 /v 1 )a 1 ǫ 2 Ŵ (0 a 1 1) Ŵ 1 2a1 = ˆV + (u 0 /v 2 )(1 2a 1 )ǫ 2 Ŵ and parameters 1 v 1 = 6(1 2t 0 t ) 2 1 = 1 2 t 0 (0 t 0 1 2 (1 1 [ ] v 2 = 1 2v 1 u 0 = 1 12 1 1 1 1 2t 0 + 6(1 2t 0 ) 3 3 )) High-order actions 09 p.11/28

Chin Action (t 0, a 1 ) (II) Explicitly, ρ T0A1 (R α,r α+1 ) = ( m ) ( ) 9N/2 3N/2 1 2π 2 ǫ 2t 2 1 t 0 ǫ N i=1 dr αa dr αb exp { m 2 2 ǫ [ 1 (r α,i r αa,i ) 2 + 1 (r αa,i r αb,i ) 2 + 1 ] (r αb,i r α+1,i ) 2 t 1 t 1 2t 0 N (v 1 V (r α,ij ) + v 2 V (r αa,ij ) + v 1 V (r αb,ij )) i<j ǫ 3 u 0 2 m N ( a1 F α,i 2 + (1 2a 1 ) F αa,i 2 + a 1 F αb,i 2)} i=1 K. Sakkos, J. Casulleras, and J. B., arxiv:0903.2763 High-order actions 09 p.12/28

Chin Action (t 0, a 1 ) (II) Schematically, e ǫĥ e v 1ǫŴa 1e t 1 ǫ ˆTe v 2ǫŴ1 2a 1e t 1 ǫ ˆTe v 1ǫŴa 1e 2t 0 ǫ ˆT High-order actions 09 p.12/28

PIMC Estimators Properties of the system are calculated using statistical estimators which use the stochastic variables of the p.d.f. generated by the Metropolis method O = 1 N s N s i=1 O(R i ) Total energy (thermodynamic): E/N = (1/N Z) Z/ β Kinetic energy (thermodynamic): K/N = (m/n βz) Z/ m Potential energy: V/N = E/N K/N In general, for any operator O(R), O(R) = 1 β 1 Z(V ) dz(v + λo) dλ λ=0 High-order actions 09 p.13/28

Sampling in PIMC Simplest method: bead a bead + movement of the center of mass of the polymer High-order actions 09 p.14/28

Sampling in PIMC Simplest method: bead a bead + movement of the center of mass of the polymer But... slowing down problems for long chains High-order actions 09 p.14/28

Sampling in PIMC Simplest method: bead a bead + movement of the center of mass of the polymer But... slowing down problems for long chains Smart collective movements are necessary to eliminate the slowing down in the sampling We use the staging method, which allows for an exact sampling of the free action (harmonic bead-bead couplings) ρ 0 (x i, x i+1 ; ǫ)...ρ 0 (x i+j 1, x i+j ; ǫ) = ( ) 1/2 m exp[ 2π 2 m ] jǫ 2 2 jǫ (x i x i+j ) 2 j 2 k=0 ( mk ) 1/2 [ exp 2π 2 m k ǫ 2 2 ǫ (x i+k+1 x i+k+1) 2] High-order actions 09 p.14/28

Chin Action: optimization... Coming back to the Chin s approximation for the action, we need to work on a previous step = Optimization of the parameters t 0 and a 1 High-order actions 09 p.15/28

Chin Action: optimization... Coming back to the Chin s approximation for the action, we need to work on a previous step = Optimization of the parameters t 0 and a 1 1D Harmonic Oscillator T = 0.1 a 1 = 0.33 t 0 = 0.09, 0.10,...,0.15 (from top to bottom) E 0.50680 0.50679 0.50678 0.50677 0.50676 0.00 0.05 0.10 0.15 1/M The zero-slope curve is crossed! High-order actions 09 p.15/28

Optimization (II) Harmonic oscillator (T = 0.1) 0.53 0.52 a 1 = 0.33 0.50004 0.50002 a 1 =0.00 a 1 =0.14 a 1 =0.25 a 1 =0.33 a 1 =0.45 E 0.51 t 0 = 0.1215 E 0.50000 0.50 0.49998 0.49 0.49996 0.48 0.05 0.10 0.15 0.20 t 0 0.00 0.05 0.10 0.15 0.20 0.25 1/M Isotime curves (= number of beads) Optimal values: PRE 71, 056703 (2005) a 1 = 0.33 a1 = 0.00 t 0 = 0.1430 a1 = 0.14 t 0 = 0.0724 a1 = 0.25 t 0 = 0.1094 a1 = 0.33 t 0 = 0.1215 a1 = 0.45 t 0 = 0.1298 High-order actions 09 p.16/28

Results for different actions Harmonic oscillator (T = 0.2) 0.5068 0.5066 E 0.5064 0.5062 0.5060 0.0 0.1 0.2 0.3 0.4 0.5 1/M PA (M = 512) TIA (M = 128) Chin-t 0 (M = 6) Chin-(t 0,a 1 ) (M = 4) High-order actions 09 p.17/28

Results for different actions H 2 drop with N = 22 (T = 6 K) 18 E/N(K) 20 22 24 0 0.004 0.008 0.012 0.016 ε(k 1 ) Chin-t 0 TIA PA High-order actions 09 p.17/28

Results for different actions Liquid 4 He (T = 5.1 K) -2.8-3 E/N -3.2-3.4-3.6 0 0.02 0.04 0.06 0.08 0.1 1/M PA (M = 512) TIA (M = 128) Chin-t 0 (M = 20) Chin-(t 0,a 1 ) (M = 14) High-order actions 09 p.17/28

Results for more exigent problems... 22 7 23 7.5 E E/N 24 8 25 0 0.004 0.008 0.012 8.5 0 0.005 0.01 0.015 0.02 1/M 1 / M H 2 drop with 22 molecules T = 1.0 K Liquid 4 He T = 0.8 K The lines correspond to 6th order fits: E/N = (E/N) 0 + A(1/M) 6 High-order actions 09 p.18/28

Computational efficiency Cost per Reduction # Performance bead beads factor PA 1.0 1 1.0 TIA 2.9 4 1.4 Chin-t 0 4.8 38 7.9 Chin-(t 0, a 1 ) 7.2 58 8.0 The computational cost per bead increases appreciably, but this increase is largely compensated for the sizeable decrease of the number of beads required to reach the asymptote ǫ 0 High-order actions 09 p.19/28

Symmetrization: sampling of permutations At very low temperatures T T c it is necessary to introduce the correct quantum statistics High-order actions 09 p.20/28

Symmetrization: sampling of permutations At very low temperatures T T c it is necessary to introduce the correct quantum statistics For bosons the action must be symmetric ρ B (R 0,R 1 ; β) = 1 N! X ρ(r 0, PR 1 ; β) Sampling the permutation space produces longer polymeric chains which are formed by more than one particle: SUPERFLUIDITY P 1 0.5 0-0.5-1 -1.5-2 0 0.5 1 1.5 2 2.5 3 3.5 4 High-order actions 09 p.20/28

Permutations Sampling over all the possible paths and connections Care has to be taken to ensure the achievement of equilibrium during the time of a simulation (many atoms involved) To take into account correctly the periodic boundary conditions to have always continuous paths D. M. Ceperley, Rev. Mod. Phys. 67, 271 (1995) High-order actions 09 p.21/28

Proposing a pair permutation... Initially the two atoms are separated A staging chain is constructed connecting the bead J of atom 1 with the bead J+m of atom 2 A staging chain is constructed connecting the bead J of atom 2 with the bead J+m of atom 1 High-order actions 09 p.22/28

Efficiency in the permutation sampling Over all the trial permutations only 5% are accepted (free-action test) and therefore sampled Over all the permutations sampled only 1% are accepted by Metropolis = Very low efficiency The length of the staging chain (joining different particles) is selected for maximizing the ratio of Metropolis-accepted permutations per real time unit Permutations involving more than 3 or 4 polymers are extremely difficult to appear,... but they are important for a correct estimation of the superfluid density High-order actions 09 p.23/28

New proposal: Worm Algorithm Proposed for Prokof ev, Boninsegni and Svistunov for PIMC in the grand canonical ensemble. Key ingredient: An open chain (worm) is introduced in the simulation. By the swap operation, long permutations are in practice achieved. Specially useful for the estimation of the superfluid density and the one-body density matrix. High-order actions 09 p.24/28

Results for bulk 4 He 6.0 6.5 E/N(K) 7.0 7.5 8.0 8.5 0 0.01 0.02 0.03 ε(k 1 ) ε dependence at T = 0.8 K Efficiency of Chin action is conserved after symmetrization High-order actions 09 p.25/28

Results for bulk 4 He 2 3 4 E/N (K) 5 6 7 8 0 1 2 3 4 T (K) Expt PIMC CHIN Energy as a function of temperature, crossing T λ High-order actions 09 p.25/28

Results for bulk 4 He 4 Expt PIMC 3 C v 2 1 0 0 1 2 3 4 T (K) Specific heat close to T λ High-order actions 09 p.25/28

Results for bulk 4 He 1.2 Expt DMC PIMC g(r) 0.8 0.4 0.0 0 2 4 6 8 10 r(å) Two-body radial distribution function High-order actions 09 p.25/28

Results for bulk 4 He 1.6 1.2 PIMC DMC Expt S(k) 0.8 0.4 0.0 0 1 2 3 4 5 6 k(å 1 ) Static structure factor High-order actions 09 p.25/28

Results for bulk 4 He 1 0.8 N=32 N=64 N=128 ρ(r)/ρ 0.6 0.4 0.2 0 0 1 2 3 4 5 6 7 8 9 r(å) One-body density matrix T = 2 K: n 0 = 5.5 % High-order actions 09 p.25/28

Small 4 He drops 4 2 E/N (K) 0 2 4 0 1 2 3 T (K), N = 20;, N = 40;, N = 70 High-order actions 09 p.26/28

Small 4 He drops ρ(r) (Å 3 ) 0.02 0.01 T=0.00 T=0.25 T=0.75 T=1.25 T=1.75 T=2.00 T=2.25 0 5 10 15 r(å) N = 40 High-order actions 09 p.26/28

Small 4 He drops ρ(r) (Å 3 ) 0.03 0.02 0.01 T=0.00 T=0.25 T=0.75 T=1.25 T=1.75 T=2.00 T=2.25 0 5 10 15 r(å) N = 70 High-order actions 09 p.26/28

Conclusions The action (t 0, a 1 ) has been used for the first time in PIMC and has shown a 6th order efficiency, not only in model problems but in real and more exigent systems ( 4 He, H 2 ) With respect to the Takahashi-Imada approximation, the new action does not require any additional derivative of the potential Migrating a TIA code to a Chin one is rather easy since the basic routines are the same In spite of substituting a bead by three beads, the efficiency of the staging corresponds to the one of a time step ǫ High-order actions 09 p.27/28

Conclusions Easier, general and with a more clear dependence with ǫ than the pair action approximation (Ceperley) The introduction of the worm allows for a better sampling of permutations = best results for n 0 and ρ s /ρ This is our choice for finite-temperature simulations in quantum fluids... High-order actions 09 p.28/28