Questions tagged [computational-physics]

Questions with this tag should be about computational physics, which is the study of physical situations with the use of software (commercial or in-house). Please note that details of writing and/or debugging code is OFF-TOPIC and should be asked at either Computational Science, Code Review or Stack Overflow.

331 questions with no upvoted or accepted answers
Filter by
Sorted by
Tagged with
8
votes
1answer
314 views

Computational scaling of quantum and classical Monte Carlo algorithms

How does the computational complexity of finding an equilibrium thermal state for a given Hamiltonian at a given temperature scale with system size under classical and quantum Monte Carlo? I know ...
7
votes
2answers
976 views

Interpretation of Stiffness Matrix and Mass Matrix in Finite Element Method

I would like to have a general interpretation of the coefficients of the stiffness matrix that appears in FEM. For instance if we are solving a linear elasticity problem and we modelize the relation ...
6
votes
0answers
79 views

Do exactly solvable stat mech systems admit efficient algorithms for finite sizes?

I come from a background in statistical mechanics (not algorithm design or complexity theory), and the following question occurred to me that I could use some expert help in beginning to understand. ...
5
votes
0answers
97 views

Do systems of fermions take longer to equilibrate than systems of bosons for complexity-theoretic reasons?

This excellent paper by Scott Aaronson persuasively argues that computational complexity can be relevant for physical processes. In particular, what's hard for a hypothetical Turing machine to do may ...
5
votes
0answers
595 views

Numerical Solution of the Propagation-Dispersion equation

I have asked this question on Computational Science and also on Mathoverflow, but no satisfactory answers so far. I thought maybe the physics community could shed some insight on the issue. I am ...
5
votes
0answers
1k views

An explanation for the Landauer's principle

Has anyone understood the Landauer's principle? What is the current status? In specific, is there a theoretical derivation of the Landauer's Principle?(not the heuristic one based on Salizard's ...
5
votes
1answer
756 views

Ray tracing in General Relativity

I would like to find out what one would see at the Schwarzschild radius of a massive non-rotating black hole, if the black hole is surrounded by a bright ring. For that, I would place the observer at ...
4
votes
0answers
152 views

$N$-body gravity simulator: why does energy conservation break down when introducing an adaptive timestep?

I am playing with an N-body gravity simulator using the velocity-verlet algorithm. In the actual simulation I normalize everything so that it is nicely behaved numerically, but I'll express things in ...
4
votes
0answers
106 views

Textbooks on algorithms for the perturbative calculation of High energy physics

For the perturbative calculation of High energy physics, I have known some packages such as FeynArts, FeynCalc, MadGraph, CompHEP, GiNaC, and so on. But I am wondering whether there exists a textbook ...
4
votes
0answers
2k views

Numerical problem in solving the Bogoliubov de Gennes equations- methods to solve?

I am trying to solve an assignment on solving the Bogoliubov de Gennes equations self-consistently in Matlab. BdG equations in 1-Dimension are as follows:- $$\left(\begin{array}{cc} -\frac{\hbar^{2}}...
3
votes
0answers
58 views

Implications of thermodynamic inconsistency in CFD calculations

During my PhD work I had to use tabulated values of thermodynamic properties of gases in some Computational Fluid Dynamics (CFD in short) simulations. CFD simulations consist in the numerical solution ...
3
votes
1answer
88 views

Choosing initial condition for Hamilton-Jacobi PDE from initial $x$ and $p$

For separable solutions to Hamilton-Jacobi PDE (say in 2D), we treat the Hamilton's principal function $S$ as $$S= W(x) + W(y) - E*t$$ and treat the separate parts as constants and find $W(x)$, $W(y)$...
3
votes
0answers
88 views

Calculate the matrix size for ground states BEC using difference with Gross–Pitaevskii equation

The Ground states BEC at $0$ temperature can be described by Gross–Pitaevskii equation as $(-\frac{\hbar^2}{2m}\nabla^2+V(r)+g|\psi|^2)\psi(r)=\mu\psi(r)$ We limit the BEC in 2D where $V(r)=\frac{1}{...
3
votes
1answer
80 views

Coupling Navier-Stokes and stochastic models for particle tracking in micro-scale free convection?

I have been using a commercially available software to simulate laminar free convection in a specific small domain (let's use channel w/ heated lower wall as an example). The scale is approx 50-100 ...
3
votes
0answers
69 views

What are common methods to remove discretization on experimental measures?

I have some experimental data, and since the measurements are digital, the data is discretized/quantized/rounded/truncated. I need to know if there are general methods, or common methods, to "smooth" ...
3
votes
0answers
57 views

Unexpected behaviour in numerical simulation of polymer

I noticed some strange behaviour when simulating the behaviour of a polymer subjected to an external pulling force at both ends, and would like to better understand its origin. I used LAMMPS to ...
3
votes
1answer
168 views

Solving the Schrödinger equation numerically on a non-uniform grid

I am currently developing a numerical solver, which solves the standard, one-dimensional, time-independent Schrödinger equation $$\frac{-\hbar^2}{2m}\frac{d^2\psi}{dx^2} + V(x)\psi = E\psi \tag{1}$$ ...
3
votes
0answers
194 views

Calculating 2 particle Partial Trace for Density Matrix in Zeeman basis for a large number of Spins

I want to trace out all spins but 2 from a density matrix in the zeeman basis for N spins. For N=3 for example I would have the basisvectors: $ |S=1.5, m=-1.5\rangle =|000\rangle, |S=1.5, m=1.5\rangle ...
3
votes
0answers
304 views

Simulation of plasma in tokamak

I am reading some papers on numerical algorithms for simulation of plasma in the context of nuclear fusion in a tokamak. I am getting a little lost as there is a huge number of references, and it is ...
3
votes
0answers
49 views

The definition of fidelity for fermion

The definition of fidelity for two mixed ensembles is $F=Tr\sqrt{\sqrt{\rho_1}\rho_2\sqrt{\rho_1}}$. Now I came across a problem in numerical calculation, Systems A,B are identical, but attached to ...
3
votes
0answers
84 views

How are boundary consitions implemented correctly in time dependent hydrodynamics?

I posted this question more than one year ago and got an answer recently. This answer looks good to me, but indicates that something is wrong in my original approach to the problem. Can someone tell ...
3
votes
0answers
83 views

Open-source code for computing response functions

Summing Feynman diagrams to compute the response functions of a microscopic model is common in many areas of physics. While conceptually straightforward, the task can be computationally intensive. ...
3
votes
0answers
393 views

How to calculate dispersion relation from a Finite Difference (FD) wave simulation

I have a python code that calculates the solution of the inhomogeneous acoustic wave equation for a 2D medium with any velocity and source configuration. It was implemented using Finite Differences ...
3
votes
1answer
74 views

Comparing two versions of the same hydrodynamic code and their error

I have two versions of a hydrodynamic code that has the same underlying physics; let's call them code A and B. However, code B is more optimized and more object oriented than A. I was trying to ...
3
votes
1answer
991 views

Determing Velocity of Moons

I have a question that I believe is relatively easy to answer, I am working on an $N$-body simulation of a fictional star system and am having trouble finding the velocity of moons so that they will ...
3
votes
0answers
210 views

black body simulation

black body radiation is typically understood from Planck's argument of light resonance in a box, from which the density of states is computed. Now, suppose I want to simulate a black body ...
3
votes
0answers
150 views

Non-linear Wave Equation - Numerical Methods

Motivation: I'm working with a highly non-linear spherical wave-like equation (second order PDE). The equation can be written on the form $$\ddot{u} = f(t, \dot{u},\dot{u}',u',u'')$$ where $'=\frac{...
3
votes
0answers
942 views

Is Bremermann's limit redundant with Landauer's limit for all practical purposes?

Trying to understand the physical limits to computation, I notice that among these we have two types of limits that constrain the minimum allowable energy for a computation. Limits that constrain the ...
3
votes
2answers
8k views

How to calculate the velocity of fluid at the outlet when density and the pressure drop are known?

I have a U- like pipe. Its inlet has atmospheric pressure $p_o=10^{5} \, Pa$. Vacuum is applied to the other end with a pressure gradient $\nabla p_v=-30 \cdot 10^{3} \, kPa/s$. The total time of the ...
2
votes
0answers
29 views

Modal filtering with Vandermonde matrix - Artificial viscosity

I am trying to implement a 2D shock detector for an artificial viscosity model to control strong nonlinearities in compressible fluids. The method I am relying on is originally from: Yu, M. L., ...
2
votes
0answers
24 views

How do I know that the time-step I'm using in my Verlet integrator is small enough?

I am performing a molecular dynamics simulation of a many-body system using the Verlet algorithm in an implicit solvent, i.e. adding noise to the equation of motion via a thermostat term. I want to ...
2
votes
1answer
65 views

Is there a program or a website able to perform all Wick contractions for a given expression?

Imagine I have an expression of the type: $$\langle \phi_{x_1} \phi_{x_1} \phi_{x_2} \phi_{x_2} \phi_{z_1} \phi_{z_1} A_{z_1} \phi_{z_2} \phi_{z_2} A_{z_2} \phi_{z_3} \phi_{z_3} A_{z_3} \phi_{z_4} \...
2
votes
0answers
59 views

Finite differences as a variational method

I think I should be able to derive finite differences for the Schroedinger equation by starting from a variational method. More specifically: in finite differences we approximate $$\{\psi(x),x\in\...
2
votes
0answers
20 views

Combined effects of strain and doping on the electronic structure of semiconductors

I've read papers in DFT studies on band gap tuning in semiconductors and the usually studied methods are either through doping or application of external strain. But its always just one or the other. ...
2
votes
0answers
53 views

A preconditioner for self-consistent iteration

I tried to derive a preconditioner for self-consistent iteration similar to section IX in arXiv:0804.2583. For simplicity, consider here only one orbital (one or two electrons) systems. Suppose that ...
2
votes
0answers
24 views

Excited state probability decay in the Monte-Carlo wave function method

When using the Monte-Carlo wave function method to simulate spontaneous decay in a two-level system one typically uses a non-Hermitian Hamiltonian, e.g.: $$H_\mathrm{tot} = H_\mathrm{sys} - i\Gamma ...
2
votes
0answers
28 views

How to set the number of fermions in the whole system in fermionic-DMRG program?

In infinite DMRG (density matrix re-normalization group) algorithm, we increase size of super-block by two sites in each iteration. How do we set number of fermions in the system? Let's say we want to ...
2
votes
1answer
58 views

Scaling Problem with Variational Method

$\def\braket#1{\langle#1\rangle}$ I am attempting to solve a particular Hamiltonian by variational method. The wavefunction that I have selected is as follows: $$ \Psi = Ne^{\frac{-kr}{2}}\sum_{i=0}...
2
votes
0answers
58 views

Negative Eigenvalues of the Hessian

I am calculating the eigenvalues of the Hessian for a ferromagnetic system. My energy has the zeeman term, a nearest neighbor exchange term, and a dipole-dipole term. I create the hessian where my ...
2
votes
0answers
33 views

Looknig for resources on finding periodic orbit and stability on multidimensional Hamiltonian systems

I am looking for resources (books, papers, algorithms, codes) that explicitly explain the computation and analysis (using the monodromy matrix) of periodic orbits of multidimensional Hamiltonian ...
2
votes
0answers
97 views

How to numerically calculate Fisher zeros?

The main quantity in the study of dynamical quantum phase transition (DQPT) is Loschmidt echo amplitude defined as $G(t)=\langle \Psi_{0}|\Psi_{0}(t)\rangle=\langle \Psi_{0}|e^{-iHt}|\Psi_{0}\rangle$...
2
votes
1answer
89 views

Sidereal time calculation from orbital elements and mean anomaly

I'm following the instructions given here for calculating the apparent position of the planets in the sky and I've come to calculating the sidereal time in section 1.8. The formula for calculating ...
2
votes
0answers
101 views

Defining the renormalized coupling constant in a lattice phi fourth simulation

I'm currently studying a scalar quantum field theory with a $\lambda\phi^4$ interaction (commonly referred to as $\phi^4$ theory. To study this theory non perturbatively I've written a program in ...
2
votes
0answers
105 views

Which test function (under variational method of approximation) should be used for a linear potential?

There is this problem, it is not for evaluation, I mean, it is not homework. There is a 3-dimension potential $V(r)=C . r$, where $r$ is the radial coordinate and $C$ is a real constant (I think it ...
2
votes
1answer
134 views

Numerical solutions for time-dependent Hamiltonian

Currently I am facing the problem to solve numerically the following equation for a double well harmonic potential: $$i\hbar \frac{\partial}{\partial t}\psi(x,t)= -\frac{\hbar}{2m}\frac{\partial^2}{\...
2
votes
0answers
104 views

How to define a non-uniform mangetic field in Geant4 simulation?

I am getting stuck in creating a gradient magnetic field by Fieldmap method in example of field04 (geant4/extend/field/field04). At the moment, I am doing OK with creating a uniform one by creating a ...
2
votes
0answers
148 views

Time rescaling in overdamped Langevin simulation

I'm simulating a system according to the Langevin equation (with inertia), however my friction coefficient is high enough that I am essentially in the overdamped regime on the timescales of one ...
2
votes
1answer
65 views

Where can I read about thermodynamics of computation?

I'm a physics undergraduate student taking a statistical and thermal physics course. Somehow, I came across Maxwell's demon and the violation of second law. However, the article involved logical ...
2
votes
0answers
49 views

Is there a way to calculate Renormalization Flow numerically?

I have an interesting system, which possess a phase transition. I would like to apply renormalization group method on this system to find the RG flow, and critical exponent. The system is governed by ...