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Quantum Dynamics of Four-Atom Reactions within the Real Wave Packet Framework Stephen K. Gray Chemistry Division Argonne National Laboratory Argonne, Illinois 60439 [email protected]

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Page 1: Quantum Dynamics of Four-Atom Reactions within the Real Wave Packet Framework Stephen K. Gray Chemistry Division Argonne National Laboratory Argonne, Illinois

Quantum Dynamics of Four-Atom Reactions within the Real Wave Packet

Framework

Quantum Dynamics of Four-Atom Reactions within the Real Wave Packet

Framework

Stephen K. GrayChemistry Division

Argonne National LaboratoryArgonne, Illinois 60439

[email protected]

Page 2: Quantum Dynamics of Four-Atom Reactions within the Real Wave Packet Framework Stephen K. Gray Chemistry Division Argonne National Laboratory Argonne, Illinois

AcknowledgementsAcknowledgements

Gabriel Balint-Kurti: co-developer of the RWP method

Evelyn Goldfield: co-developer of the four-atom implementation

Page 3: Quantum Dynamics of Four-Atom Reactions within the Real Wave Packet Framework Stephen K. Gray Chemistry Division Argonne National Laboratory Argonne, Illinois

OutlineOutline

• Introductory Remarks

• Real Wave Packet Framework:• Cosine Iterative Equation

• Modified Schrödinger Equation

• Inferring Physical Observables

• Four-Atom Systems:• Representation

• Dispersion Fitted Finite Differences

• Initial Conditions and Final State Analysis

• Cross Sections and Rate Constants

• Concluding Remarks

Page 4: Quantum Dynamics of Four-Atom Reactions within the Real Wave Packet Framework Stephen K. Gray Chemistry Division Argonne National Laboratory Argonne, Illinois

Introductory RemarksIntroductory Remarks

• Real wave packet (RWP) method: An approach for obtaining accurate quantum dynamics involving the real part of a wave packet and Chebyshev iterations [Gray and Balint-Kurti]

• Can view it as a highly streamlined version of Tal-Ezer and Kosloff’s propagator

• Shares features with: Mandelshtam and Taylor’s Chebyshev expansion of the Green’s operator, Kouri and co-workers’ “time-independent” wave packets, Chen and Guo’s Chebyshev propagator

Page 5: Quantum Dynamics of Four-Atom Reactions within the Real Wave Packet Framework Stephen K. Gray Chemistry Division Argonne National Laboratory Argonne, Illinois

Cosine Iterative EquationCosine Iterative Equation

ihddtΨ(t) = HΨ(t)

Ψ(t+τ) = exp(−iHτ/h) Ψ(t)

Ψ(t−τ) = exp(+iHτ/h) Ψ(t) implies

Ψ(t+τ) = − Ψ(t−τ) + 2 cos(Hτ/h) Ψ(t)

Page 6: Quantum Dynamics of Four-Atom Reactions within the Real Wave Packet Framework Stephen K. Gray Chemistry Division Argonne National Laboratory Argonne, Illinois

Ψ(t) = Q(t) + i P(t)

If H is time-independent,

Q(t+τ) = − Q(t−τ) + 2 cos(Hτ/h) Q(t)

Including absorption, A:

Q(t+τ) = A −AQ(t−τ) + 2 cos(Hτ/h) Q(t)[ ]

Page 7: Quantum Dynamics of Four-Atom Reactions within the Real Wave Packet Framework Stephen K. Gray Chemistry Division Argonne National Laboratory Argonne, Illinois

• Cosine equation was successful,

S. K. Gray, J. Chem. Phys. 96, 6543 (1992)

• However, cos(H) must still be evaluated

in some way

• Can we do better?

Page 8: Quantum Dynamics of Four-Atom Reactions within the Real Wave Packet Framework Stephen K. Gray Chemistry Division Argonne National Laboratory Argonne, Illinois

Modified Schrödinger EquationModified Schrödinger Equation

ihd

duχ (u) = f (H )χ (u)

• Underlying time-independent Schrödinger equationhas the same bound states (and scattering states)

• Solutions of the modified equation contain the sameinformation as the more standard one

Page 9: Quantum Dynamics of Four-Atom Reactions within the Real Wave Packet Framework Stephen K. Gray Chemistry Division Argonne National Laboratory Argonne, Illinois

χ(u) = q(u) + i p(u)

q(u+δ) = A −Aq(u−δ) + 2 cos(f(H)δ/h) q(u)[ ]Let

f(H) = −hδ cos−1Hs( )

Hs = as H + bs

Then the “cosine” equation loses its cosine :

q(u+δ) = A −Aq(u−δ) + 2 Hs q(u)[ ]

Page 10: Quantum Dynamics of Four-Atom Reactions within the Real Wave Packet Framework Stephen K. Gray Chemistry Division Argonne National Laboratory Argonne, Illinois

Inferring Physical ObservablesInferring Physical Observables

Φ(E) = c(E) 12πh dt exp(iEt/h) Ψ(t)

−∞

+∞∫ = c(E) δ(E−H) Ψ(0)

Use

δ(E−H) =df(E)dE δf(E)−f(H)[ ]

To obtain a connection to f(H) dynamics :

Φ(E) = c(E) df(E)dE δf(E)−f(H)[ ] Ψ(0)

= c(E) df(E)dE 1

2πh du expif(E)u/h[ ] χ(u)−∞

+∞∫

Page 11: Quantum Dynamics of Four-Atom Reactions within the Real Wave Packet Framework Stephen K. Gray Chemistry Division Argonne National Laboratory Argonne, Illinois

But χ(u) is still complex -- how to relate to

q(u) = Re[χ(u)] ?

du exp if (E)u / h[ ]−∞

+∞

∫ χ (u) = 2 du exp if (E)u / h[ ]−∞

+∞

∫ q(u)

If χ has no f(E) components for f(E) < 0 (or f(E) > 0)

Allows energy-resolved scattering and related quantities,e.g., S matrix elements and reaction probabilities, to beobtained from Fourier analysis of q.

Page 12: Quantum Dynamics of Four-Atom Reactions within the Real Wave Packet Framework Stephen K. Gray Chemistry Division Argonne National Laboratory Argonne, Illinois

If Ψ(t) satisfies

ihddtΨ(t) = HΨ(t)

and χ(u) satisfies

ihdduχ(u) = f(H)χ(u)

Then if each have the same initial condition, Taylor seriesexpansion of f(H) shows that

Ψ(t) ≈ expiα t/β[ ] χu=t/β[ ]

with

β(E ) = ashδ 1−E s2

Let u = k δ then physical time and Chebyshev iteration k arerelated by

t ≈ ash k1−E s2

Page 13: Quantum Dynamics of Four-Atom Reactions within the Real Wave Packet Framework Stephen K. Gray Chemistry Division Argonne National Laboratory Argonne, Illinois

Four-Atom SystemsFour-Atom Systems

Diatom-diatom

Jacobi coordinates,

body-fixed z-axis

is the R vector

θ1

R

θ2r2

r1 ϕ

AB + CD ABC + D

Page 14: Quantum Dynamics of Four-Atom Reactions within the Real Wave Packet Framework Stephen K. Gray Chemistry Division Argonne National Laboratory Argonne, Illinois

RepresentationRepresentation

qJ ,K ,p(R,r1 ,r2 ,θ1 ,θ1 ,ϕ ,k) =

C j1 ,k1 , j2J ,K ,p

j1 ,k1 , j2

∑ R,r1 ,r2 ,k( ) G j1 ,k1 , j2J ,K ,p θ1 ,θ 2 ,ϕ( )

J = total angular momentum quantum numberp = parityK = projection of total angular momentum on a body-fixed axis (often an approximately good quantum number)

Page 15: Quantum Dynamics of Four-Atom Reactions within the Real Wave Packet Framework Stephen K. Gray Chemistry Division Argonne National Laboratory Argonne, Illinois

G j1 ,k1 , j2J ,K ,p

Gatti and co-workers; Goldfield; Chen and Guo

Note: Most applications so far have assumed Kto be good (centrifugal sudden approximation)

Gj1,k1,j2J,K,p : Parity-adapted angular functions –

linear superpositions of allowed “primitive” functions based on (J, K), (j1 ,k1) and (j2, k2),

K = k1 + k2.

Page 16: Quantum Dynamics of Four-Atom Reactions within the Real Wave Packet Framework Stephen K. Gray Chemistry Division Argonne National Laboratory Argonne, Illinois

H and H qH and H q

ˆ H = −h22μ

∂2∂R2 −h2

2m1∂2∂r12

−h22m2

∂2∂r22

+ (̂ J − ˆ j 1 − ˆ j 2)22μR2 + ˆ j 12

2m1r12 + ˆ j 22

2m2r22 + V(R,r1,r2,θ1,θ2,ϕ)

H q = T q + V q = (Td + Trot) q + V q

Page 17: Quantum Dynamics of Four-Atom Reactions within the Real Wave Packet Framework Stephen K. Gray Chemistry Division Argonne National Laboratory Argonne, Illinois

Comments on H q :Comments on H q :

• Three distance (or radial) kinetic energy

contributions evaluated with either

dispersion fitted finite differences (DFFD’s) or potential-optimized discrete-variable representations (PODVR’s)

DFFD: Gray and Goldfied

PODVR: Echave and Clary; Wei and

Carrington

Page 18: Quantum Dynamics of Four-Atom Reactions within the Real Wave Packet Framework Stephen K. Gray Chemistry Division Argonne National Laboratory Argonne, Illinois

DFFD’sDFFD’s

Can obtain signifcantly betterAccuracy than standard FDapproximation

Error in reaction probability for 3D D + H2 reaction

Page 19: Quantum Dynamics of Four-Atom Reactions within the Real Wave Packet Framework Stephen K. Gray Chemistry Division Argonne National Laboratory Argonne, Illinois

V q V q

v j1 , j2 ,k1( ) = q j1 , j2 ,k1( )

v j1 , j2 ,k1( ) T1 ⏐ → ⏐ v θ1 , j2 ,k1( )

v θ1 , j2 ,k1( ) T2 ⏐ → ⏐ v θ1 ,θ 2 ,ϕ( )

v θ1 ,θ 2 ,k1( ) Tϕ ⏐ → ⏐ v θ1 ,θ 2 ,ϕ( )

v θ1 ,θ 2 ,ϕ( ) = V θ1 ,θ 2 ,ϕ( ) v θ1 ,θ 2 ,ϕ( )

v θ1 ,θ 2 ,ϕ( ) Tϕ

−1

⏐ → ⏐ ⏐ v θ1 ,θ 2 ,k1( )

v θ1 ,θ 2 ,ϕ( ) T2−1

⏐ → ⏐ ⏐ v θ1 , j2 ,k1( )

v θ1 , j2 ,k1( ) T1−1

⏐ → ⏐ ⏐ v j1 , j2 ,k1( )

Basis to grid, multiplyBy diagonal V, thenConvert back to basis

A key “trick” thatallows large rotationalbases to be treated

Favorable, near linearscaling with problemsize

Page 20: Quantum Dynamics of Four-Atom Reactions within the Real Wave Packet Framework Stephen K. Gray Chemistry Division Argonne National Laboratory Argonne, Illinois

Propagation and AnalysisPropagation and Analysis

qk+1( ) = A −Aqk−1( ) + 2 Hs q(k)[ ] , k = 0, 1, …

χ(k=0) = Gε,σ(R) ψv1(r1) ψv2(r2) Gj1,j2,k1J,p θ1,θ2,ϕ( )

Gε,σ(R) = complex-valued incoming Gaussian wave packet

q(k=0) = Reχ(0)[ ]q(k=1) = Hs q(0) − 1−Hs2 Im χ(0)[ ]

Page 21: Quantum Dynamics of Four-Atom Reactions within the Real Wave Packet Framework Stephen K. Gray Chemistry Division Argonne National Laboratory Argonne, Illinois

Reaction ProbabilitiesReaction Probabilities

PI E( ) = ΦI(E) ˆ F ΦI(E)

ˆ F = h2msiδ(s−s0)∂∂s −

∂∂sδ(s−s0)

⎡ ⎣ ⎢ ⎤

⎦ ⎥

Write ΦI as FT of q (Meijer et al.) -- problem reduces to saving certain dq/ds and q at s0 as a function of effectivetime and then constructing PI afterwards

Page 22: Quantum Dynamics of Four-Atom Reactions within the Real Wave Packet Framework Stephen K. Gray Chemistry Division Argonne National Laboratory Argonne, Illinois

Cross Sections, Rate ConstantsCross Sections, Rate Constants

Since we can compute PI(E), I = some initial state, there is nothing special about

constructing cross sections and rate constants

The problem is the large number of I states

that must be considered: I = J, p, K, j1, j2,

k1, v1, v2

Page 23: Quantum Dynamics of Four-Atom Reactions within the Real Wave Packet Framework Stephen K. Gray Chemistry Division Argonne National Laboratory Argonne, Illinois

A State-Resolved Cross Section:A State-Resolved Cross Section:

σv1 , j1 ,v2 , j2 (ε ) = π

2με (2J +1) Pv1 , j1 ,v2 , j2

J

J

∑ ε( )

Pv1 , j1 ,v2 , j2J ε( ) =

1

(2 j1 +1)(2 j2 +1) Pv1 , j1 ,k1 ,v2 , j2

J ,K ,p ε( )K ,p,k1

Page 24: Quantum Dynamics of Four-Atom Reactions within the Real Wave Packet Framework Stephen K. Gray Chemistry Division Argonne National Laboratory Argonne, Illinois

Rate ConstantsRate Constants

kT( ) = gel(T)Qr(T) gj1,j2exp[−εv1,j1,v2,j2/(kBT)]v1,j1,v2,j2

∑ kv1,j1,v2,j2(T)

kv1,j1,v2,j2T( ) = 8kBTπμ ⎛ ⎝ ⎜ ⎞

⎠ ⎟1/2 1

(kT)2 dε ε exp−ε/(kBT)[ ]0

∞∫ σv1,j1,v2,j2(ε)

Qr(T) = gj1,j2exp[−εv1,j1,v2,j2/(kBT)]v1,j1,v2,j2∑

Page 25: Quantum Dynamics of Four-Atom Reactions within the Real Wave Packet Framework Stephen K. Gray Chemistry Division Argonne National Laboratory Argonne, Illinois

Approximation: J-ShiftingApproximation: J-Shifting

pv1,j1,v2,j2ε() ≡ Pv1,j1,v2,j2Jref ε()

Pv1,j1,v2,j2J ε() ≅ pv1,j1,v2,j2ε' = ε + (EJref−EJ) ⎡

⎣ ⎢ ⎤ ⎦ ⎥

with

EJ = 1nJ B J(J+1)+(A−B )K2( ) nJKK∑

Use result for a “reference” J to extrapolate to other J

Page 26: Quantum Dynamics of Four-Atom Reactions within the Real Wave Packet Framework Stephen K. Gray Chemistry Division Argonne National Laboratory Argonne, Illinois

Bowman has extensively discussed J-shiftingThe idea of using non-zero J values to base the J-shifting is not new -- previous work along related lines includes

• S. L. Mielke, G. C. Lynch, D. G. Truhlar, and D. W. Schwenke, Chem. Phys. Lett. 216, 441 (1993).

• H. Wang, W. H. Thompson, and W. H. Miller, J. Phys. Chem. A 102, 9372 (1998).

• J. M. Bowman and H. M. Shnider, J. Chem. Phys. 110, 4428 (1999).

• D. H. Zhang and J. Z. H. Zhang, J. Chem. Phys. 110, 7622 (1999).

Page 27: Quantum Dynamics of Four-Atom Reactions within the Real Wave Packet Framework Stephen K. Gray Chemistry Division Argonne National Laboratory Argonne, Illinois

Concluding RemarksConcluding Remarks

For accurate quantum dynamics of three and four-atom systems, the RWP method is a good choice of methods -- less memory and more efficient than comparable complex wave packet calculations

Page 28: Quantum Dynamics of Four-Atom Reactions within the Real Wave Packet Framework Stephen K. Gray Chemistry Division Argonne National Laboratory Argonne, Illinois

However, to go beyond four-atoms requires (most likely) abandoning the detailed scattering theory

approach involving complicated angular momentum bases and detailed state-resolved considerations

Cumulative reaction probability and related approaches to direct evaluation of averaged quantities (Miller, Manthe)

The use of parallel computers and Cartesian coordinates?

Page 29: Quantum Dynamics of Four-Atom Reactions within the Real Wave Packet Framework Stephen K. Gray Chemistry Division Argonne National Laboratory Argonne, Illinois