ISING_2D_SIMULATION
Monte Carlo 2D Ising Model

ISING_2D_SIMULATION is a FORTRAN90 program which carries out a Monte Carlo simulation of a 2D Ising model, using GNUPLOT to create graphics images of the initial and final configurations.

A 2D Ising model is defined on an MxN array of cells. Each cell can have a "charge" or "spin" of +1 or -1. A neighborhood of a cell is defined to be itself, and the four immediate neighbors to the north, south, east, and west. A cell may change its charge; the likelihood of doing so depends on whether the charge of the neighbors agrees with the charge of the cell.

This program begins with a random assignment of charges to the cells. It then computes the neighbor agreement at each cell. It then updates the entire configuration in one step, using the neighbor agreement to determine the likelihood of flipping.

Currently, the chance that a cell will "flip" is determined from a table of probabilities based on the number of agreeing neighbors:

Agree	1	2	3	4	4
Prob	0.98	0.85	0.50	0.15	0.02

Usage:

ising_2d_simulation m n iterations thresh seed

• m and n are the number of rows and columns of cells. The default values are 10.
• iterations is the number of iterations to take. The default value is 15.
• thresh is a number between 0 and 1 that chooses the percentage of negative (red) charges. The default value is 0.5.
• seed is a seed for the random number generator. The default value is 123456789.

Note that the user might also want to vary the transition probabilities. This cannot currently be done on the command line, and requires editing the program and recompiling it.

Licensing:

The computer code and data files described and made available on this web page are distributed under the GNU LGPL license.

Languages:

ISING_2D_SIMULATION is available in a C version and a C++ version and a FORTRAN90 version and a MATLAB version.

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Reference:

1. American National Standard for Programming Language: Fortran - Extended,
   American National Standards Institute, 1992,
   pages 296-299.

Source Code:

• ising_2d_simulation.f90, the source code.

Examples and Tests:

• ising_2d.txt, the output file, using the default options.
• ising_2d_initial.txt, GNUPLOT commands to plot the initial configuration.
• ising_2d_initial.png, a plot of the initial configuration.
• ising_2d_final.txt, GNUPLOT commands to plot the final configuration.
• ising_2d_final.png, a plot of the final configuration.

List of Routines:

• MAIN is the main program for ISING_2D_SIMULATION.
• CH_CAP capitalizes a single character.
• CH_EQI is a case insensitive comparison of two characters for equality.
• CH_TO_DIGIT returns the integer value of a base 10 digit.
• GET_INPUT gets input parameters.
• ISING_2D_AGREE returns the number of neighbors agreeing with each cell.
• ISING_2D_INITIALIZE initializes the Ising array.
• ISING_2D_STATS prints information about the current step.
• NEIGHBOR_2D_STATS prints neighbor statistics about the current step.
• R8MAT_UNIFORM_01 fills an R8MAT with unit pseudorandom numbers.
• S_TO_I4 reads an integer value from a string.
• S_TO_R8 reads an R8 value from a string.
• TIMESTAMP prints the current YMDHMS date as a time stamp.
• TRANSITION carries out a Monte Carlo simulation of a 3D Ising model.

You can go up one level to the FORTRAN90 source codes.