FORTRAN variable output not equal to input - random

Question

First of all I am a complete novice to FORTRAN, and most forms of programming in general. With that said I am attempting to build a box, then randomly generate x, y, z coordinates for 100 atoms. From there, the goal is to calculate the distance between each atom and perform some math on the distance result. Below is my code. Even though n is defined as 100, and will print '100', when I print cx I only get 20 results.
program energytot
implicit none
integer :: i, n, j, seed(12), k, m
double precision:: sigma, r, epsilon, lx, ly, lz
double precision, dimension(:), allocatable :: cx, cy, cz, dx, dy, dz, x, y, z, LJx, LJy, LJz
allocate(x(n), y(n), z(n), LJx(n), LJy(n), LJz(n), dx(n), dy(n), dz(n))
n = 100 !Number of molecules inside the box
sigma = 4.1
epsilon = 1.7
!Box length with respect to the axis
lx = 15
ly = 15
lz = 15
do i=1,12
seed(i)=1+3
end do
!generate n random numbers for x, y, z
call RANDOM_SEED(PUT = seed)
call random_number(x)
call random_number(y)
call random_number(z)
!convert random numbers into x, y, z coordinates with (0,0,0) as the central point
cx = ((2*x)-1)*(lx*0.5)
cy = ((2*y)-1)*(lx*0.5)
cz = ((2*z)-1)*(lz*0.5)
do j=1,n-1
do k=j+1,n
dx = ABS((cx(j) - cx(j+1)))
LJx = 4 * epsilon * ((sigma/dx(j))**12 - (sigma/dx(j))**6)
dy = ABS((cy(j) - cy(j+1)))
LJy = 4 * epsilon * ((sigma/dy(j))**12 - (sigma/dy(j))**6)
dz = ABS((cz(j) - cz(j+1)))
LJz = 4 * epsilon * ((sigma/dz(j))**12 - (sigma/dz(j))**6)
end do
end do
print*,cx
print*,x
end program energytot

Answer 1

You declare cx (and cy and cz) allocatable, but you do not allocate space for them. Moreover, before you assign a value to variable n, you use it as the number of elements to allocate for your other allocatables. Why do any of those even need to be dynamically allocated in the first place?
I would replace this code ...
integer :: i, n, j, seed(12), k, m
double precision:: sigma, r, epsilon, lx, ly, lz
double precision, dimension(:), allocatable :: cx, cy, cz, dx, dy, dz, x, y, z, LJx, LJy, LJz
allocate(x(n), y(n), z(n), LJx(n), LJy(n), LJz(n), dx(n), dy(n), dz(n))
n = 100 !Number of molecules inside the box
... with this:
integer, parameter :: n = 100
integer :: i, j, seed(12), k, m
double precision :: sigma, r, epsilon, lx, ly, lz
double precision, dimension(n) :: cx, cy, cz, dx, dy, dz, x, y, z, LJx, LJy, LJz
I also observe that in the loop where you compute distances, you loop over variable k, but you do not use its value. As a result, it looks like you compute the same distances many times over.