Path integral formulation

The Path integral formulation 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 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)

$Q(\beta ,V)=\int {\mathrm {d} }x_{1}\int _{x_{1}}^{x_{1}}Dx(\tau )e^{-S[x(\tau )]}$

where $S[x(\tau )]$ is the Euclidian action, given by ( Eq. 2)

$S[x(\tau )]=\int _{0}^{\beta \hbar }H(x(\tau ))$

where $x(\tau )$ is the path in time $\tau$ and $H$ is the Hamiltonian. This leads to ( Eq. 3)

$Q_{P}=\left({\frac {mP}{2\pi \beta \hbar ^{2}}}\right)^{P/2}\int ...\int {\mathrm {d} }x_{1}...{\mathrm {d} }x_{P}e^{-\beta \Phi _{P}(x_{1}...x_{P};\beta )}$

where the Euclidean time is discretised in units of

$\varepsilon ={\frac {\beta \hbar }{P}},P\in {\mathbb {Z} }$

$x_{t}=x(t\beta \hbar /P)$

$x_{P+1}=x_{1}$

and ( Eq. 4)

$\Phi _{P}(x_{1}...x_{P};\beta )={\frac {mP}{2\beta ^{2}\hbar ^{2}}}\sum _{t=1}^{P}(x_{t}-x_{t+1})^{2}+{\frac {1}{P}}\sum _{t=1}^{P}V(x_{t})$.

where $P$ is the Trotter number. In the Trotter limit, where $P\rightarrow \infty$ these equations become exact. In the case where $P=1$ 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 $x_{t}$ interacts with is neighbours $x_{t-1}$ and $x_{t+1}$ via a harmonic spring. The second term provides the internal potential energy. Thus in three dimensions one has the density operator

${\hat {\rho }}(\beta )=\exp \left[-\beta {\hat {H}}\right]$

which thanks to the Trotter formula we can tease out $\exp \left[-\beta (U_{\mathrm {spring} }+U_{\mathrm {internal} })\right]$, where

$U_{\mathrm {spring} }={\frac {mP}{2\beta ^{2}\hbar ^{2}}}\sum _{t=1}^{P}|\mathbf {r} _{t}-\mathbf {r} _{t+1}|^{2}$

and

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

The internal energy is given by

$\langle U\rangle ={\frac {3NP}{2\beta }}-\langle U_{\mathrm {spring} }\rangle +\langle U_{\mathrm {internal} }\rangle$

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

$\langle K_{P}\rangle ={\frac {3NP}{2\beta }}-\langle U_{\mathrm {spring} }\rangle$

Harmonic oscillator

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

$\rho (x',x)={\sqrt {\frac {m\omega }{2\pi \hbar \sinh \omega \beta \hbar }}}\exp \left(-{\frac {m\omega }{2\hbar (\sinh \omega \beta \hbar )^{2}}}\left((x^{2}+x'^{2})\cosh \omega \beta \hbar -2xx'\right)\right)$

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, $\beta$ 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 ($d$) rotational degrees of freedom the Hamiltonian can be written in the form ( Eq. 2.1):

${\hat {H}}={\hat {T}}^{\mathrm {translational} }+{\hat {T}}^{\mathrm {rotational} }+{\hat {V}}$

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

$T^{\mathrm {rotational} }=\sum _{i=1}^{d^{\mathrm {rotational} }}{\frac {{\hat {L}}_{i}^{2}}{2\Theta _{ii}}}$

where ${\hat {L}}_{i}$ are the components of the angular momentum operator, and $\Theta _{ii}$ are the moments of inertia. For a rigid three dimensional asymmetric top the kernel is given by ( Eq. 5):

$\rho (\omega ,\omega ';\beta /P)=\sum _{JM{\tilde {K}}}\left({\frac {2J+1}{8\pi ^{2}}}\right)d_{MM}^{J}({\tilde {\theta '}})\cos \left[M({\tilde {\phi }}'+{\tilde {\chi }}')\right]\left|A_{{\tilde {K}}M}^{JM}\right|^{2}\exp \left(-{\frac {\beta }{P}}E_{\tilde {K}}^{JM}\right)$

where $\omega$ are the Euler angles, $d_{MM}^{J}$ is the Wigner D-matrix and $E_{\tilde {K}}^{JM}$ are the eigenenergies.

Techniques

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)

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, 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

• R. P. Feynman "Statistical Mechanics", Benjamin, Reading, Massachusetts, (1972) ISBN 0-201-36076-4 Chapter 3.
• 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.