编辑代码

program main
	integer :: i,a,b,nn,ll,nst,H,x,y
    integer, allocatable :: spin(:)
    real(8) :: Z,T,k
    real(8), allocatable :: W(:)

    k=1 !!玻尔兹曼常数
    Z=0
    ll=2 !!长度
    nn=ll*ll
    nst=2**nn !!2的n次方个态，指数用双星号
    T=2.00 !!温度
    allocate (spin(0:nn-1))
    allocate (W(0:nst-1))
    do a=0,nst-1
        H=0
        do i=0,nn-1
            if (btest(a,i)==true) then
                spin(i)=1
            end if
            else if (btest(a,i)==false) then
                spin(i)=-1
            end if
        enddo
        do i=0,nn-1
            x=mod(i,ll)
            y=i/ll
            s1=spin(mod(x+1,ll)+y*ll)
            s2=spin(x+mod(y+1,ll)*ll)
            s3=spin(mod(x-1+ll,ll)+y*ll)
            s4=spin(x+mod(y-1+ll,ll)*ll)
            H=(-1)*spin(i)*(s1+s2+s3+s4)+H
        enddo
        H=H/2
        W(a)=exp(-H/k/T)
        Z=Z+W(a)
    enddo
    print *, "H:", H
    print *, W
end