For use with An Introduction to Thermal Physics by Daniel V. Schroeder.

This code is written in True Basic. This language is especially easy to use for graphical simulations such as this, but has unfortunately been in decline for a number of years. The current version is supported only for the Windows operating system. A free demo version is available. If you have a very old computer, you can still use these free versions for Macintosh (classic) and DOS.

If you don't like (or can't use) True Basic, click here for a list of versions of the Ising program in other languages.

A few comments: Unlike the pseudocode in the text, this version of the program runs indefinitely until you press the mouse button. To modify it to run for a fixed number of iterations, remove the "do . . . loop" lines and the "get mouse" instruction, then change the value 100 in the "for" instruction to whatever number you like. Also note that this version prompts you for the lattice size and temperature, so you don't have to change the code every time you run the program. Feel free to change "blue" and "yellow" in the colorsquare subroutine to your two favorite colors. Enjoy!

program ising ! Monte Carlo simulation of a 2D Ising model ! using the Metropolis algorithm (importance sampling) dim s(50,50) ! declare array of dipoles input prompt "Size of lattice (max 50): ": size input prompt "Temperature (units of epsilon/k): ": T set window 0,(size+2)*1.55,0,(size+2)*1.1 ! coordinates for graphics randomize ! different random numbers for each run call initialize(s,size) do ! main iteration loop for iteration = 1 to 100 let i = int(rnd*size+1) ! choose a random row number let j = int(rnd*size+1) ! and a random column number call deltaU(s,i,j,size,Ediff) ! compute hypothetical delta-U of flip if Ediff <= 0 then ! if flipping reduces the energy... let s(i,j) = -s(i,j) ! then flip it! call colorsquare(i,j,s(i,j)) else if rnd < exp(-Ediff/T) then ! otherwise the Boltzmann factor let s(i,j) = -s(i,j) ! gives the probability of flipping call colorsquare(i,j,s(i,j)) end if end if next iteration get mouse x,y,mousestate ! check mouse every 100 iterations loop until mousestate = 1 end sub deltaU(s(,),i,j,size,Ediff) ! compute delta-U of flipping a dipole ! (complicated because of pbc) if i=1 then let top = s(size,j) else let top = s(i-1,j) if i=size then let bottom = s(1,j) else let bottom = s(i+1,j) if j=1 then let left = s(i,size) else let left = s(i,j-1) if j=size then let right = s(i,1) else let right = s(i,j+1) let Ediff = 2*s(i,j)*(top+bottom+left+right) end sub sub initialize(s(,),size) ! initialize to random array for i = 1 to size for j = 1 to size if rnd < .5 then let s(i,j) = 1 else let s(i,j) = -1 call colorsquare(i,j,s(i,j)) next j next i end sub sub colorsquare(i,j,s) ! color square i,j according to s if s = 1 then set color "blue" else set color "yellow" box area i,i+1,j,j+1 end sub

*Last modified on January 20, 2013.*