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.

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17 views

Is QCD computable?

Are arbitrarily good approximations of the time evolution of any QCD system, given initial and boundary conditions, Turing computable? Can lattice QCD simulations be used to do so in theory? How ...
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14 views

Correct method for error propagation in pseudo-data approach using model

I want to compute the error on the integral over experimental data $(y_i,\delta y_i)$ in a pseudo-data approach using model ($x_i$ are assumed to be known error-less numbers). If I were using direct ...
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How to calculate optical conductivity from numerical eigenstates of tight-binding model?

Let's say we have a 1D spatially inhomogeneous tight-binding model that does not have momentum as a good quantum number. We can numerically diagonalize it to get the spectrum and eigenstates. But how ...
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33 views

Simulating Frisbee trajectories in Python [on hold]

So I've been trying to simulate the trajectory of a frisbee in flight thrown with different angles of attack/initial velocities. The differential equations used for the code are: $$\frac{dv_y}{dt} = ...
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28 views

Software for 3D magnetic field simulation [closed]

What are some user friendly softwares for making customized 3D models and doing magnetic field simulation. I have used FEMM for 2D models and it works great but is not capable of doing 3D models.
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17 views

Squared matrix elemet differs between FORM/MadGraph [closed]

I would like to consider a process like e+ e- -> Z -> t t~ and compute the squared matrix element. My code in FORM (only the relevant pieces) is the following: ...
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33 views

Software to simulate molten salt flow and thermodynamic operations

I was curious if there was any software (preferably in C++, Java, and/or python) that could be used to simulate the following: Heat capacity of a fluid Heat transfer through a liquid and a solid ...
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1answer
30 views

Optical Logic Gates using Polarisers

I am trying to build an optical logic circuit using the horizontal or vertical orientations of light as the two binary states and laser as the source. Before I ask you my question I wanted to first ...
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35 views

(Finite Element Method) Zero thermal conductivity and Non-zero convection boundary condition

I asked this same question in Computational Science Stack but no one proved it mathematically (see the link below). Link of original question: https://scicomp.stackexchange.com/questions/32959/fem-1d-...
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20 views

Fluid rendering technique (C++/3D) [closed]

I'd like to render fluid based on density value. Let's say I have a cube with 8 points (each of thoses points has its proper density value). So, If i want represent the fluid form on this cubic ...
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37 views

Numerically solving unbounded Fokker-Planck equation

I am wanting to solve a 3D Fokker-Planck equation of the form: $$\partial_{t}p(\mathbf{x}, t) = -\nabla \cdot \mathbf{J}$$ where $\mathbf{J} = \mathbf{v}(\mathbf{x})p(\mathbf{x}, t) - D\nabla p(\...
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28 views

What methods can be used to interpolate values of a gradient?

Let's say I have a cubic mesh, the points of which I have obtained a vector value for, thus forming this discrete vector space of sort, I'm not really sure of the proper terminology for this which ...
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11 views

Finit element modelling of rigid bodies and machining

I would like to computer model of material machining. Is finite element metod suitable for this? I am imagining 3D grid of voxels, which are tracking matter properties across space. Is this correct ...
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36 views

Quantum Monte Carlo Loop Algorithm for quantum spin: why is the freezing graph present in ferromagnetic Ising model?

I study the loop algorithm (Evertz et al). I cannot understand, why the freezing graph type where we have to flip all 4 spins together is not present for the quantum-XY model and the anti-/...
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51 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\...
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Results concerning computation of differential cross section of one (two) particle scattering in non-relativistic QM

I have developed a method of numerically computing the differential cross section $f(\textbf{p'} \leftarrow \textbf{p})$ for an arbitrary time independent potential $V(\textbf{r})$ in 3D. Since two ...
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1answer
68 views

Modelling lunar motion in Python [closed]

I'm trying to solve the equations of motion of the Moon in the Moon-Sun-Earth system (equations 1139-1141 here:http://farside.ph.utexas.edu/teaching/336k/Newtonhtml/node130.html) My initial ...
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1answer
74 views

How can I use Runge-Kutta4 to solve this orbit in polar coordinates?

I want to simulate a simple orbit of a planet moving around a star which is fixed in position. I have formulated the ODEs for this problem using Lagrangian Mechanics and have found the equations of ...
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1answer
22 views

Scattered fields in surface plasmon excitation with grating

We need to use prism-coupling (e.g. Kretschmann or Otto configuration) or use grating to excite surface plasmon between metal and dielectric. What I understand is that when we use a grating or ...
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0answers
10 views

sizes of monolayer supercell used in density functional theory calculations

I want to save computational time in a DFT study on monolayer Mo$\textrm{S}_2$ so I'm looking to use a $2\times2$ or a $3\times2$ monolayer supercell so that the calculations would only involve 24 to ...
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0answers
17 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. ...
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0answers
32 views

Lippmann-Schwinger Equation (Numerical Method)

I recently tried to code up the numerical solution for solving the LS equation, which is based on the "scatt.py" code from R.H. Landau's "Computational Physics". The formulation is kind of like: $$ R_{...
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13 views

What are possible causes that can lead a self consistent calculation to diverge in DFT?

I am extending a code written to do self consistent Density Functional Theory calculations to the case of spin polarized systems. Due to the modifications, the calculations are leading to diverging ...
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1answer
63 views

Discontinuities in a Poincare map for a double pendulum

I'm generating poincare sections of a double pendulum, and they mostly look okay, but some of them have weird discontinuities that seem wrong. The condition for these sections is the standard $\...
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2answers
92 views

How to actually find a Hartree-Fock ground state?

I am interested in finding the Hartree-Fock ground state for a system of interacting fermions (with totally local scattering, so a delta-function interaction potential). I have read through some ...
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1answer
33 views

Metropolis Algorithm Transition-Proposal Probability

I'm working my way through a short section on the Metropolis algorithm in the lecture notes on Computational Quantum Physics by Prof. Troyer. However, I am not sure what probability distribution was ...
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0answers
48 views

Symplectic time-integrator for Fokker-Planck equation

Is there a way to use a symplectic time-integrator for the numerical solution Fokker-Planck equation of the form: $$\partial_{t}p = -\nabla \cdot (\underline{v}p - D\nabla p)~?$$ where $p = p(\...
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1answer
49 views

Physics Graphic Simulations

I am interested in modeling physics using graphics simulations. I took a python course with CS majors and one of the assignments was to simulate a planet orbiting a star. That being said, I was ...
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0answers
23 views

Why do the Binder Cumulants of different system sizes intersect at the critical point?

When Monte Carlo simulations are performed for spin models (Ising model etc.) the critical temperature can be found by simulating for different lattice sizes and plotting the Binder Cumulant for them. ...
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0answers
37 views

How do I calculate the pressure on the walls of my MD simulation?

So I've built a simulation in vpython where I have multiple hard balls that collide with each other and with the walls of the cube I've placed them in. I then replaced the values for their radius and ...
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0answers
22 views

Density of states LL in graphene [closed]

I am using the Kernel Polynomial Method to determine the spectral density of a 2DEG system that has been sujected to a perpendicular magnetic field B. I wish to determine (a) What the amplitudes of ...
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1answer
38 views

Numerical calculation of skyrmion number

I'm having trouble finding a numerical method to calculate the skyrmion number for a certain system. I want to calculate $$ n = \frac{1}{4\pi}\int \textbf{M} \cdot \left( \frac{\partial \textbf{M}}{\...
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0answers
43 views

$<P_T>$ distribution as function of PL

I have a conceptual problem when working with some Monte Carlo generated data. I have to simulate the $P_T$ and $P_L$ distribution for $N$ particles through Monte Carlo methods as follows: $$\theta \...
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1answer
17 views

Obtain element boundary forces in Finite Element Method

Suppose that we have a rectangular domain discretized using 16 voxels (4x4 voxels) and I apply a Finite Element Analysis with arbitrary boundary conditions in the domain. As a result, I obtain the ...
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0answers
25 views

Winner-takes-it-all algorithm in particle decays

How does the winner-takes-it-all algorithm work for two competing processes, specifically in particle decay processes? What are its advantages against veto algorithm, for example?
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1answer
219 views

How do we find the equation for the gyrating motion of a particle in a uniform magnetic field and a non-uniform Electric field? [closed]

Considering the gyrating motion is not negligible and also retaining the guiding center drift, how do we get the trajectories x(t),y(t),z(t) of the particle? In this case is the variation in the ...
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0answers
21 views

FDTD simulations for metamaterials and complex refractive index

I have designed a working FDTD code to simulate electromagnetic wave propagation using Yee's algorithm. I have been able to place materials such as glass slabs, etc which have positive and real ...
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0answers
24 views

About likelihood ratio computation

My head is stuck into a rather simple problem I don't know why. I'll explain: 1- bosons have an angular distribution in $\theta$ and $\phi$: $$F(\theta, \phi) = \frac{3}{4\pi}[0.5(1-\alpha) + (0.5)(3\...
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1answer
61 views

Double pendulum Poincaré section issues

I'm generating high-resolution Poincaré sections of a double pendulum, and I'm running into some issues in creating the initial conditions for a given section. In general, I describe my pendulum with ...
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1answer
44 views

Analyticity of the initial data

Consider an initial data $(q_{i j},K_{i j})$ on a Cauchy hypersurface $\Sigma$ in a globally hyperbolic spacetime. How reasonable is it to impose the condition that these functions are analytic? I am ...
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1answer
33 views

Program for simplifying expressions of many-body Pauli operators

I have a system of $N$ spins, with the usual $\sigma_i^{x,y,z}$ operators defined on each site (operators at different sites commute). I would like to find a program, perhaps in Mathematica, with the ...
2
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1answer
65 views

Choosing initial condition for Hamilton-Jacobi PDE

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 $W(x)$, $W(y)$...
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1answer
104 views

Master equation for the mechanical modes

Consider the standard model of optomechanical systems with a single optical cavity mode coupled to a mechanical oscillator, which is canonically modeled as a FP cavity with one fixed mirror and one ...
2
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0answers
50 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 ...
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1answer
46 views

Implementing the Hartree-Fock method in two dimensions from scratch

I am interesting in writing a complete code for the Hartree-Fock method to improve my understanding of it. It seems relatively complex in three dimensions, so I am wondering if it would be simpler in ...
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1answer
59 views

How to calculate the spin correlation function for the 2d Potts model? [closed]

I basically want to make a C code for the same but I am not able to figure out how to index the array to store correlation values in terms of distance, because the distance can be non integer in the ...
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1answer
59 views

Resource recommendation for machine learning

I am planing to change my field (in PhD) and learn Machine Learning to differentiate different phases of strongly correlated matter. I learned Monte Carlo method in my MS and have intermediate level ...
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1answer
76 views

Question about code units and physical units (hydrodynamics simulations)

I'm working on a code that implements smoothed particle hydrodynamics (SPH) method for solving the equations of magnetohydrodynamics (MHD) with self-gravity. In research papers regarding existing ...
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0answers
22 views

Swap protocol in parallel tempering MCMC simulations

Consider a parallel-tempering Markov-Chain Monte Carlo (MCMC) simulation of a system, with inverse temperatures $\beta_1 < \cdots < \beta_N$ and energies $E_1, \cdots, E_N$. If there are two ...