First of all I am a complete novice to FORTRAN. 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, which becomes the value "r" of the Lennard-Jones potential energy equation. Then calculate the LJ potential, and finally sum the potential of the entire box. A previous question that I had asked about this project is here. The problem is that I get the same calculated value over and over and over again. My code is below.
program energytot
implicit none
integer, parameter :: n = 100
integer :: i, j, k, seed(12)
double precision :: sigma, r, epsilon, lx, ly, lz
double precision, dimension(n) :: x, y, z, cx, cy, cz
double precision, dimension(n*(n+1)/2) :: dx, dy, dz, LJx, LJy, LJz
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)=i+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
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(k)))
LJx = 4 * epsilon * ((sigma/dx(j))**12 - (sigma/dx(j))**6)
dy = ABS((cy(j) - cy(k)))
LJy = 4 * epsilon * ((sigma/dy(j))**12 - (sigma/dy(j))**6)
dz = ABS((cz(j) - cz(k)))
LJz = 4 * epsilon * ((sigma/dz(j))**12 - (sigma/dz(j))**6)
end do
end do
print*, dx
end program energytot
What exactly is your question? What do you want your code to do, and what does it do instead?
If you're having problems with the final print statement print*, dx, try this instead:
print *, 'dx = '
do i = 1, n * (n + 1) / 2
print *, dx(i)
end do
It seems that dx is too big to be printed without a loop.
Also, it looks like you're repeatedly assigning the array dx (and other arrays in the loop) to a single value. Try this instead:
i = 0
do j=1,n-1
do k=j+1,n
i = i + 1
dx(i) = ABS((cx(j) - cx(k)))
end do
end do
This way, each value cx(j) - cx(k) gets saved to a different element of dx, instead of overwriting previously saved values.