Dynamical perturbation theory for the evaluation of the relaxation function

1989 ◽  
Vol 67 (1) ◽  
pp. 31-36 ◽  
Author(s):  
I. M. Kim ◽  
Bae-Yeun Ha
Keyword(s):  
Perturbation Theory ◽  
Relaxation Function ◽  
Continued Fraction ◽  
Recurrence Relations ◽  
Unperturbed System ◽  
Many Body ◽  
Analytic Expression ◽  
Infinite Continued Fraction ◽  
Many Body Systems

Use of the infinite continued-fraction representation has played an important role in the evaluation of the relaxation function of many-body systems. By using the method of recurrence relations, we propose here a dynamical perturbation theory for the evaluation of the infinite continued fraction. The analytic expression of the relaxation function is given in terms of the known relaxation function for the unperturbed system. Two illustrative examples are also given.

Dimensionality dependence of dynamical correlations: exact results from a quantum many body system

1993 ◽  
Vol 441 (1911) ◽  
pp. 169-179 ◽  
Keyword(s):  
Low Temperatures ◽  
Relaxation Function ◽  
Mean Field ◽  
Exact Results ◽  
Body System ◽  
Many Body ◽  
Many Body Systems

Using one of the simplest model interacting quantum many body systems it is shown that the relaxation function behaves remarkably differently at low temperatures depending upon whether the lattice dimensionality of the system, D < ∞ or → ∞. The results illustrate the possible limitations of mean-field descriptions of dynamics at T < T c in quantum many body systems.

Method of Recurrence Relations and Applications to Many-Body Systems

1987 ◽  
Vol T19B ◽  
pp. 498-504 ◽  
Author(s):  
M Howard Lee ◽  
J Hong ◽  
J Florencio
Keyword(s):  
Recurrence Relations ◽  
Many Body ◽  
Many Body Systems

scholarly journals Recent Advances in the Calculation of Dynamical Correlation Functions

2020 ◽  
Vol 8 ◽  
Author(s):  
J. Florencio ◽  
O. F. de Alcantara Bonfim
Keyword(s):  
Correlation Functions ◽  
Relaxation Function ◽  
Heisenberg Model ◽  
Recurrence Relations ◽  
Dynamic Properties ◽  
Approximation Schemes ◽  
Xy Model ◽  
Many Body ◽  
Dynamical Correlation ◽  
Numerical Work

We review various theoretical methods that have been used in recent years to calculate dynamical correlation functions of many-body systems. Time-dependent correlation functions and their associated frequency spectral densities are the quantities of interest, for they play a central role in both the theoretical and experimental understanding of dynamic properties. In particular, dynamic correlation functions appear in the fluctuation-dissipation theorem, where the response of a many-body system to an external perturbation is given in terms of the relaxation function of the unperturbed system, provided the disturbance is small. The calculation of the relaxation function is rather difficult in most cases of interest, except for a few examples where exact analytic expressions are allowed. For most of systems of interest approximation schemes must be used. The method of recurrence relation has, at its foundation, the solution of Heisenberg equation of motion of an operator in a many-body interacting system. Insights have been gained from theorems that were discovered with that method. For instance, the absence of pure exponential behavior for the relaxation functions of any Hamiltonian system. The method of recurrence relations was used in quantum systems such as dense electron gas, transverse Ising model, Heisenberg model, XY model, Heisenberg model with Dzyaloshinskii-Moriya interactions, as well as classical harmonic oscillator chains. Effects of disorder were considered in some of those systems. In the cases where analytical solutions were not feasible, approximation schemes were used, but are highly model-dependent. Another important approach is the numericallly exact diagonalizaton method. It is used in finite-sized systems, which sometimes provides very reliable information of the dynamics at the infinite-size limit. In this work, we discuss the most relevant applications of the method of recurrence relations and numerical calculations based on exact diagonalizations. The method of recurrence relations relies on the solution to the coefficients of a continued fraction for the Laplace transformed relaxation function. The calculation of those coefficients becomes very involved and, only a few cases offer exact solution. We shall concentrate our efforts on the cases where extrapolation schemes must be used to obtain solutions for long times (or low frequency) regimes. We also cover numerical work based on the exact diagonalization of finite sized systems. The numerical work provides some thermodynamically exact results and identifies some difficulties intrinsic to the method of recurrence relations.

Automatic computing of the grand potential in finite temperature many-body perturbation theory

2019 ◽  
Vol 30 (11) ◽  
pp. 1950100
Author(s):  
M. A. Tag ◽  
M. E. Mansour
Keyword(s):  
Perturbation Theory ◽  
Finite Temperature ◽  
Mathematical Expression ◽  
Thermodynamic Quantities ◽  
Many Body ◽  
Textual Data ◽  
Grand Potential ◽  
Many Body Perturbation Theory ◽  
Analytic Expression ◽  
Text Language

A new program created in C/C[Formula: see text] language generates automatically the analytic expression of grand potential and prints it in Latex2e format and in textual data. Another code created in Mathematica language can translate the textual data into a mathematical expression and help any interested to evaluate the thermodynamic quantities in analytic or numeric forms.

The relaxation function of the spin van der Waals model in a weak magnetic field

1990 ◽  
Vol 68 (1) ◽  
pp. 49-53 ◽  
Author(s):  
Y. K. Lee ◽  
I. M. Kim
Keyword(s):  
Magnetic Field ◽  
Relaxation Function ◽  
Weak Magnetic Field ◽  
Continued Fraction ◽  
Van Der Waals ◽  
Exact Result ◽  
Van Der Waals Model ◽  
Perturbative Method ◽  
Infinite Continued Fraction

We consider the spin van der Waals model in a weak magnetic field. For this model we obtain the relaxation function by applying the perturbative method that was developed for the evaluation of the relaxation function represented as an infinite continued fraction. To show the applicability of the perturbative method, we make comparisons between an exact result and various degrees of approximation.

Divergence of Many-Body Perturbation Theory for Noncovalent Interactions of Large Molecules

2019 ◽  
Author(s):  
Brian Nguyen ◽  
Guo P Chen ◽  
Matthew M. Agee ◽  
Asbjörn M. Burow ◽  
Matthew Tang ◽  
...  
Keyword(s):  
Perturbation Theory ◽  
System Size ◽  
State Density ◽  
Second Order ◽  
Binding Energies ◽  
Basis Set ◽  
Many Body ◽  
Many Body Perturbation Theory ◽  
Relative Errors ◽  
Ground State Density

Prompted by recent reports of large errors in noncovalent interaction (NI) energies obtained from many-body perturbation theory (MBPT), we compare the performance of second-order Møller–Plesset MBPT (MP2), spin-scaled MP2, dispersion-corrected semilocal density functional approximations (DFA), and the post-Kohn–Sham random phase approximation (RPA) for predicting binding energies of supramolecular complexes contained in the S66, L7, and S30L benchmarks. All binding energies are extrapolated to the basis set limit, corrected for basis set superposition errors, and compared to reference results of the domain-based local pair-natural orbital coupled-cluster (DLPNO-CCSD(T)) or better quality. Our results confirm that MP2 severely overestimates binding energies of large complexes, producing relative errors of over 100% for several benchmark compounds. RPA relative errors consistently range between 5-10%, significantly less than reported previously using smaller basis sets, whereas spin-scaled MP2 methods show limitations similar to MP2, albeit less pronounced, and empirically dispersion-corrected DFAs perform almost as well as RPA. Regression analysis reveals a systematic increase of relative MP2 binding energy errors with the system size at a rate of approximately 1‰ per valence electron, whereas the RPA and dispersion-corrected DFA relative errors are virtually independent of the system size. These observations are corroborated by a comparison of computed rotational constants of organic molecules to gas-phase spectroscopy data contained in the ROT34 benchmark. To analyze these results, an asymptotic adiabatic connection symmetry-adapted perturbation theory (AC-SAPT) is developed which uses monomers at full coupling whose ground-state density is constrained to the ground-state density of the complex. Using the fluctuation–dissipation theorem, we obtain a nonperturbative “screened second-order” expression for the dispersion energy in terms of monomer quantities which is exact for non-overlapping subsystems and free of induction terms; a first-order RPA-like approximation to the Hartree, exchange, and correlation kernel recovers the macroscopic Lifshitz limit. The AC-SAPT expansion of the interaction energy is obtained from Taylor expansion of the coupling strength integrand. Explicit expressions for the convergence radius of the AC-SAPT series are derived within RPA and MBPT and numerically evaluated. Whereas the AC-SAPT expansion is always convergent for nondegenerate monomers when RPA is used, it is found to spuriously diverge for second-order MBPT, except for the smallest and least polarizable monomers. The divergence of the AC-SAPT series within MBPT is numerically confirmed within RPA; prior numerical results on the convergence of the SAPT expansion for MBPT methods are revisited and support this conclusion once sufficiently high orders are included. The cause of the failure of MBPT methods for NIs of large systems is missing or incomplete “electrodynamic” screening of the Coulomb interaction due to induced particle–hole pairs between electrons in different monomers, leaving the effective interaction too strong for AC-SAPT to converge. Hence, MBPT cannot be considered reliable for quantitative predictions of NIs, even in moderately polarizable molecules with a few tens of atoms. The failure to accurately account for electrodynamic polarization makes MBPT qualitatively unsuitable for applications such as NIs of nanostructures, macromolecules, and soft materials; more robust non-perturbative approaches such as RPA or coupled cluster methods should be used instead whenever possible.<br>

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