Issues with setting random seed [duplicate] - random

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Random numbers keep coming out the same, despite random seed being used
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Closed last year.
I am attempting to write a Montecarlo algorithm to simulate interaction between agents in a population. This algorithm needs to call two random numbers at each iteration (say, 10^9 iterations).
My issue here is that everytime I change the seed (to obtain different realizations), the RNG is giving me the same output (same Montecarlo events). I have tried different ways of implementing it with to no avail.
I am completely new to Fortran and copying this code from MATLAB. Am I doing something wrong in the way I'm implementing this code?
Below is what I tried:
program Gillespie
implicit none
integer*8, parameter :: n_max = 10.0**8 ! max. number of iterations
integer*8 :: t_ext, I_init, S_init, jump, S_now, I_now, i, u
real*8 :: t, N, a0, tau, st, r1, r2
real, parameter :: beta = 1000
real, parameter :: gammma = 99.98
real, parameter :: mu = 0.02
real, parameter :: R0 = beta/(gammma+mu)
integer :: seed = 11
real, dimension(n_max) :: S_new ! susceptible pop. array
real, dimension(n_max) :: I_new ! infected pop. array
real, dimension(n_max) :: t_new ! time array
real, dimension(5) :: events ! events array
open(unit=3, file='SIS_output.dat')
t = 0 ! initial time
N = 40 ! initial population size
jump = 1 ! time increment (save in uniform increments)
u = 2
t_ext = 0 ! extiction time
I_init = 2 ! initial infected pop.
S_init = N-I_init ! initial susceptible pop.
S_now = S_init
I_now = I_init
S_new(1) = S_init ! initialize susceptibles array
I_new(1) = I_init ! initialize infected array
t_new(1) = t ! initialize time array
write(3,*) t_new(1), S_new(1), I_new(1) ! write i.c. to array
call random_seed(seed)
do i=2, n_max
call random_number(r1)
call random_number(r2)
events(1) = mu*N ! Birth(S)
events(2) = mu*S_now ! Death(S)
events(3) = mu*I_now ! Death(I)
events(4) = (beta*S_now*I_now)/N ! Infection
events(5) = gammma*I_now ! Recovery
a0 = events(1)+events(2)+events(3)+events(4)+events(5)
tau = LOG(1/r1)*(1/a0) ! time increment
t = t + tau ! update time
st = r2*a0 ! stochastic time???
! update the populations
if (st .le. events(1)) then
S_now = S_now + 1
else if (st .gt. events(1) .AND. st .le.
#(events(1) + events(2))) then
S_now = S_now - 1
else if (st .gt. (events(1) + events(2)) .AND. st .le.
#(events(1) + events(2) + events(3))) then
I_now = I_now - 1
else if (st .gt. (events(1) + events(2) + events(3)) .AND.
#st .le. (events(1) + events(2) + events(3) + events(4))) then
S_now = S_now - 1
I_now = I_now + 1
else
S_now = S_now + 1
I_now = I_now - 1
end if
! save time in uniform increments
if(t .ge. jump) then
t_new(u) = floor(t)
S_new(u) = S_now
I_new(u) = I_now
write(3,*) t_new(u), S_new(u), I_new(u)
jump = jump+1
u = u+1
end if
! write(3,*) t_new(i), S_new(i), I_new(i)
!N = S_now + I_now ! update population post event
if(I_now .le. 0) then ! if extinct, terminate
print *, "extinct"
goto 2
end if
end do
2 end program Gillespie
I appreciate all input. Thanks.

Answer 1

Your call
call random_seed(seed)
is strange. I thought it should not be allowed without a keyword argument, but it actually is inquiring for the size of the random seed array.
For a proper way of initializing seed see the example in
https://gcc.gnu.org/onlinedocs/gfortran/RANDOM_005fSEED.html