Path integral formulation

The path integral formulation, here from the statistical mechanical point of view, is an elegant method by which quantum mechanical contributions can be incorporated within a classical simulation using Feynman path integrals (see the additional reading section). Such simulations are particularly applicable to light atoms and molecules such as hydrogen, helium, neon and argon, as well as quantum rotators such as methane and hydrogen-bonded systems such as water. From a more idealised point of view path integrals are often used to study quantum hard spheres.

Principles

In the path integral formulation the canonical partition function (in one dimension) is written as ( Eq. 1)

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle Q(\beta, V)= \int {\mathrm d} x_1 \int_{x_1}^{x_1} Dx(\tau)e^{-S[x(\tau)]}}

where Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle S[x(\tau)]} is the Euclidian action, given by ( Eq. 2)

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle S[x(\tau)] = \int_0^{\beta \hbar} H(x(\tau))}

where Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle x(\tau)} is the path in time Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \tau} and Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle H} is the Hamiltonian. This leads to ( Eq. 3)

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle Q_P = \left( \frac{mP}{2 \pi \beta \hbar^2} \right)^{P/2} \int ... \int {\mathrm d}x_1... {\mathrm d}x_P e^{-\beta \Phi_P (x_1...x_P;\beta)}}

where the Euclidean time is discretised in units of

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \varepsilon = \frac{\beta \hbar}{P}, P \in {\mathbb Z}}

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle x_t = x(t \beta \hbar/P)}

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle x_{P+1}=x_1}

and ( Eq. 4)

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \Phi_P (x_1...x_P;\beta)= \frac{mP}{2\beta^2 \hbar^2} \sum_{t=1}^P (x_t - x_{t+1})^2 + \frac{1}{P} \sum_{t=1}^P V(x_t)} .

where Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle P} is the Trotter number. In the Trotter limit, where Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle P \rightarrow \infty} these equations become exact. In the case where Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle P=1} these equations revert to a classical simulation. It has long been recognised that there is an isomorphism between this discretised quantum mechanical description, and the classical statistical mechanics of polyatomic fluids, in particular flexible ring molecules, due to the periodic boundary conditions in imaginary time. It can be seen from the first term of the above equation that each particle Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle x_t} interacts with is neighbours $x_{t-1}$ and Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle x_{t+1}} via a harmonic spring. The second term provides the internal potential energy. The following is a schematic for the interaction between two (purple) atoms. Here we show the atoms having five Trotter slices, forming what can be thought of as a "ring polymer molecule". The harmonic springs between Trotter slices are in red, and white/green bonds represent the intermolecular pair potential.

In three dimensions one has the density operator

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \hat{\rho} (\beta) = \exp\left[ -\beta \hat{H} \right]}

which thanks to the Trotter formula we can tease out Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \exp \left[ -\beta (U_{\mathrm {spring}}+ U_{\mathrm{internal}} ) \right]} , where

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle U_{\mathrm {spring}} = \frac{mP}{2\beta^2 \hbar^2} \sum_{t=1}^P | \mathbf{r}_t - \mathbf{r}_{t+1} |^2}

and

$U_{\mathrm {internal} }={\frac {1}{P}}\sum _{t=1}^{P}V(\mathbf {r} _{t})$

The internal energy is given by

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \langle U \rangle = \frac{3NP}{2\beta}- \langle U_{\mathrm {spring}} \rangle + \langle U_{\mathrm{internal}} \rangle }

The average kinetic energy is known as the primitive estimator, i.e.

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \langle K_P \rangle = \frac{3NP}{2\beta}- \langle U_{\mathrm {spring}} \rangle }

Harmonic oscillator

The density matrix for a harmonic oscillator is given by ( Eq. 10-44)

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rho(x',x)= \sqrt{ \frac{m \omega}{2 \pi \hbar \sinh \omega \beta \hbar} } \exp \left( - \frac{m \omega}{2 \hbar (\sinh \omega \beta \hbar)^2 } \left( (x^2 + x'^2 ) \cosh \omega \beta \hbar - 2xx'\right)\right)}

Related reading

• Barry R. Holstein "The harmonic oscillator propagator", American Journal of Physics 66 pp. 583-589 (1998)
• L. Moriconi "An elementary derivation of the harmonic oscillator propagator", American Journal of Physics 72 pp. 1258-1259 (2004)

Wick rotation and imaginary time

One can identify the inverse temperature, Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \beta} with an imaginary time $it/\hbar$ (see § 2.4).

• G. C. Wick "Properties of Bethe-Salpeter Wave Functions", Physical Review 96 pp. 1124-1134 (1954)

Rotational degrees of freedom

In the case of systems having (Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle d} ) rotational degrees of freedom the Hamiltonian can be written in the form ( Eq. 2.1):

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \hat{H} = \hat{T}^{\mathrm {translational}} + \hat{T}^{\mathrm {rotational}}+ \hat{V}}

where the rotational part of the kinetic energy operator is given by ( Eq. 2.2)

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle T^{\mathrm {rotational}} = \sum_{i=1}^{d^{\mathrm {rotational}}} \frac{\hat{L}_i^2}{2\Theta_{ii}}}

where Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \hat{L}_i} are the components of the angular momentum operator, and Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \Theta_{ii}} are the moments of inertia.

Rigid rotators

Main article: Rigid top propagator

Computer simulation techniques

The following are a number of commonly used computer simulation techniques that make use of the path integral formulation applied to phases of condensed matter

Path integral Monte Carlo

Path integral Monte Carlo (PIMC)

• J. A. Barker "A quantum-statistical Monte Carlo method; path integrals with boundary conditions", Journal of Chemical Physics 70 pp. 2914- (1979)

Path integral molecular dynamics

Path integral molecular dynamics (PIMD)

• M. Parrinello and A. Rahman "Study of an F center in molten KCl", Journal of Chemical Physics 80 pp. 860- (1984)

Centroid molecular dynamics

Centroid molecular dynamics (CMD)

• Jianshu Cao and Gregory A. Voth "The formulation of quantum statistical mechanics based on the Feynman path centroid density. I. Equilibrium properties", Journal of Chemical Physics 100 pp. 5093-5105 (1994)
• Jianshu Cao and Gregory A. Voth "The formulation of quantum statistical mechanics based on the Feynman path centroid density. II. Dynamical properties", Journal of Chemical Physics 100 pp. 5106- (1994)
• Seogjoo Jang and Gregory A. Voth "A derivation of centroid molecular dynamics and other approximate time evolution methods for path integral centroid variables", Journal of Chemical Physics 111 pp. 2371- (1999)
• Rafael Ramírez and Telesforo López-Ciudad "The Schrödinger formulation of the Feynman path centroid density", Journal of Chemical Physics 111 pp. 3339-3348 (1999)
• E. A. Polyakov, A. P. Lyubartsev, and P. N. Vorontsov-Velyaminov "Centroid molecular dynamics: Comparison with exact results for model systems", Journal of Chemical Physics 133 194103 (2010)

Ring polymer molecular dynamics

Ring polymer molecular dynamics (RPMD)

• Ian R. Craig and David E. Manolopoulos "Quantum statistics and classical mechanics: Real time correlation functions from ring polymer molecular dynamics", Journal of Chemical Physics 121 pp. 3368- (2004)
• Bastiaan J. Braams and David E. Manolopoulos "On the short-time limit of ring polymer molecular dynamics", Journal of Chemical Physics 125 124105 (2006)

Contraction scheme

• Thomas E. Markland and David E. Manolopoulos "An efficient ring polymer contraction scheme for imaginary time path integral simulations", Journal of Chemical Physics 129 024105 (2008)
• Thomas E. Markland and David E. Manolopoulos "A refined ring polymer contraction scheme for systems with electrostatic interactions" Chemical Physics Letters 464 pp. 256-261 (2008)

Grand canonical Monte Carlo

A path integral version of the Widom test-particle method for grand canonical Monte Carlo simulations:

• Qinyu Wang, J. Karl Johnson and Jeremy Q. Broughton "Path integral grand canonical Monte Carlo", Journal of Chemical Physics 107 pp. 5108-5117 (1997)

Applications

• Jianshu Cao and Gregory A. Voth "Semiclassical approximations to quantum dynamical time correlation functions", Journal of Chemical Physics 104 pp. 273-285 (1996)
• C. Chakravarty and R. M. Lynden-Bell "Landau free energy curves for melting of quantum solids", Journal of Chemical Physics 113 pp. 9239-9247 (2000)

Additional reading

• P. A. M. Dirac "The Lagrangian in Quantum Mechanics", Physikalische Zeitschrift der Sowjetunion 3 pp. 64-72 (1933)
• R. P. Feynman "Statistical Mechanics", Benjamin, Reading, Massachusetts, (1972) ISBN 0-201-36076-4 Chapter 3.
• Tohru Morita "Solution of the Bloch Equation for Many-Particle Systems in Terms of the Path Integral", Journal of the Physical Society of Japan 35 pp. 980-984 (1973)
• F. W. Wiegel "Path integral methods in statistical mechanics", Physics Reports 16 pp. 57-114 (1975)
• J. A. Barker "A quantum-statistical Monte Carlo method; path integrals with boundary conditions", Journal of Chemical Physics 70 pp. 2914-2918 (1979)
• D. M. Ceperley "Path integrals in the theory of condensed helium", Reviews of Modern Physics 67 279 - 355 (1995)
• Charusita Chakravarty "Path integral simulations of atomic and molecular systems", International Reviews in Physical Chemistry 16 pp. 421-444 (1997)

External links

• Density matrices and path integrals computer code on SMAC-wiki.