fortran code do not cycle - fortran

Question

i try to solve this simple three body problem with the following code:
Program Main
Implicit real*8 (A-H,O-Z)
real*8 ome,mu, rho, R
duepi=8*datan(1.d0)
ome=1
mu=0.001
T_per=duepi/ome
rho=0.1
R=1.0
N_step=100
c Open the file
OPEN(unit=11, file="prova1.txt")
c Nested do loops
do iy0=1,100
do iP0=1,100
c Calc value for 0
y0 = real(iy0)/100.
x0 = 0
c Calc value for py0
py0 = real(iP0)/100.
px0 = 0
x=x0
y=y0
px=px0
py=py0
dt=T_per/N_step
E0=H(x,y,px,py)
k_max=100*N_step
k=0
t=0
errh=0
c---------
c start integration loop
c--------
do k=1,k_max
call sym4(x,y,px,py,dt)
E= H(x,y,px,py)
errh=abs(E-E0)
t=k*dt
enddo
do k=1,k_max
call sym4(x,y,px,py,-dt)
E= H(x,y,px,py)
errh=abs(E-E0)
t=t-dt
enddo
write(11,*) y0, py0, errh
enddo ! iP0
enddo ! iy0
close(11)
end
subroutine sym1(x,y,px,py,dt)
Implicit real*8 (A-H,O-Z)
c
call f(x,y,fx,fy)
pxnew=px+dt*fx
pynew=py+dt*fy
xnew=x+dt*pxnew
ynew=y+dt
c
x=xnew
y=ynew
px=pxnew
py=pynew
end
subroutine sym1_B(x,y,px,py,dt)
Implicit real*8 (A-H,O-Z)
c
xnew=x+dt*px
ynew=y+dt
call f(xnew,ynew,fxnew,fynew)
pxnew=px+dt*fxnew
pynew=py+dt*fynew
c
x=xnew
y=ynew
px=pxnew
py=pynew
end
subroutine f(x,y,fx,fy)
Implicit real*8 (A-H,O-Z)
real*8 ome,mu,rho,R
fx = ((1-mu)*(rho+x))/((rho*rho+2*rho*x+y*y)**(1.5)) -
& (mu*(R+x))/((R**2-2*R*x+x*x+y*y)**(1.5))
fy = ((1-mu)*(rho+x))/((rho**2+x**2+2*rho*x+y**2)**(1.5))/
& + (mu*y)/((R**2-2*R*x+x**2+y**2)**(1.5))
return
end
real*8 function H(x,y,px,py)
Implicit real*8 (A-H,O-Z)
real*8 ome,mu,rho,R
c h=px*px/2.d0+ py +(1+eps*cos(ome*y))*x*x/2
c h=px*px/2.d0+ py -(1+eps*cos(ome*y))*cos(x)
r12 = sqrt( ( (x*cos(ome*y)-y*sin(ome*y))+rho*cos(ome*y) )**2
& + ( x*sin(ome*y)+y*cos(ome*y) + rho*sin(ome*y) )**2 )
r13 = sqrt( ( (x*cos(ome*y)-y*sin(ome*y))-R*cos(ome*y) )**2
& + ( x*sin(ome*y)+y*cos(ome*y) - R*sin(ome*y) )**2 )
h=(px**2+py**2)/2.d0 - (1-mu)/r12 - mu/r13 + py
return
end
subroutine sym2(x,y,px,py,dt)
Implicit real*8 (A-H,O-Z)
call f(x,y,fx,fy)
xnew= x+ px*dt + fx*dt**2/2.d0
ynew= y+ dt ! così è giusto
call f(xnew,ynew,fxnew,fynew)
pxnew= px+ dt*(fx+fxnew )/2.d0
pynew= py+ dt*(fy+fynew )/2.d0
x=xnew
y=ynew
px=pxnew
py=pynew
end
subroutine sym4(x,y,px,py,dt)
Implicit real*8 (A-H,O-Z)
sq2=2**(1.d0/3.d0)
alpha= 1.d0/(2-sq2)
beta= sq2/(2-sq2)
dt1= dt*alpha
dt2=-dt*beta
call sym2(x,y,px,py,dt1)
call sym2(x,y,px,py,dt2)
call sym2(x,y,px,py,dt1)
return
end
the code calls sympletic integrators and solve the 3body problem. But when i try to run it, there aren't compiling errors, the output.txt file show only the initial grid and not the errh this column give me only NaN, can someone help me?
Is maybe an initial condition errors (strange initial conditions for velocities or positions, omega is wrong ...)?

Answer 1

As stated already in the comments, your program has many code style problems (wrong intentation, ...) as well as not using best Fortran practises (e.g. implicit none). Your problem can be solved by trivial usage of a debugger.
An obvious problem is that you are using uninitialized variables in the functions:
subroutine f(x,y,fx,fy) : rho, mu, R in calculation of fx and fy, which produces NaN
function H() : ome, rho, mu, ...
and similarly on other places