MC simulation code for 2D Ising model

ising2d.go

/*
MC simulation code for 2-dimensional Ising model

Copyright (C) 2014 Yusuke Konishi

This software is released under the MIT License.
http://opensource.org/licenses/mit-license.php
*/
package main

import (
	"fmt"
	"math"
	"math/rand"
)

// L is the size of the system.
// The number of Monte Carlo steps is mcs1+mcs2,
// and we use mcs2 steps for calculating magnetization.
var L, mcs1, mcs2 int = 100, 1000, 1000

func main() {
// L2 is the number of spins.
	L2 := L*L
	spin := make([]int, L2)
	im := make([]int, L)
	ip := make([]int, L)
// using seed 100
	rand.Seed(100)
// Initially, the system has random configuration.
	for i :=0; i < L2; i++ {
		spin[i] = 2 * int(2*rand.Float64()) - 1
	}
// im gives i-1, ip gives i+1, 0-1 = L-1, (L-1)+1 = 0
	for i := 0; i < L; i++ {
		im[i] = i-1
		ip[i] = i+1
	}
	im[0] = L-1
	ip[L-1] = 0
// Temperature changes from 10 to 0.1, d(temperature) = 0.1
	for itemp := 100; itemp > 0; itemp-- {
		var temp, inverse_temp, mag, mag2, mag4 float64
		temp = float64(itemp) / 10.0
		inverse_temp = 1.0 / temp
// weight gives square of the exp(-(energy)/(temperature))
		w1 := math.Exp(inverse_temp * 4)
		w2 := w1*w1
		var weight = map[int]float64 {
			-4: 1.0 / w2,
			-2: 1.0 / w1,
			0: 1.0,
			2: w1,
			4: w2,
		}
// Monte Carlo loop
		mag = 0
		mag2 = 0
		mag4 = 0
		for time := 0; time < mcs1 + mcs2; time++ {
			var sum_spin int = 0
			for i :=0; i < L2; i++ {
				var x, y int = i/L, i%L
				nearest_spins := spin[im[x]*L+y] + spin[ip[x]*L+y] + spin[x*L+im[y]] + spin[x*L+ip[y]]
				if weight[nearest_spins * -spin[i]] > rand.Float64() {
					spin[i] = -spin[i]
				}
				sum_spin = sum_spin + spin[i]
			}
			if time > mcs1 {
				mag_tmp := float64(sum_spin) / float64(L2)
				mag_tmp2 := mag_tmp * mag_tmp
				mag_tmp4 := mag_tmp2 * mag_tmp2
				mag = mag + mag_tmp
				mag2 = mag2 + mag_tmp2
				mag4 = mag4 + mag_tmp4
			}
		}
		mag = mag / float64(mcs2)
		mag2 = mag2 / float64(mcs2)
		mag4 = mag4 / float64(mcs2)
		fmt.Println(temp, mag, mag2, mag4)
	}
}