using fortran in python

Question

how can I write this fortran code in python? I need to use this subroutine inside of my homework but I am writing my code in python.Any way to use it or change it?

# This subroutine calculates Energy and forces for Lennard-Jones clusters
#  input:   nat = no. of atoms, xyz = atomic Cartesian coordinates
#  output:  epot = energy, fxyz = forces on each atom  
#======================================================================== 

subroutine LJ EandF(nat,xyz,epot,fxyz)
implicit none
integer nat
real*8 xyz(3,nat),epot,fxyz(3,nat)
real*8 dx,dy,dz,r2
real*8 t2,t6,ff
real*8, parameter :: eps=1.d0
real*8, parameter :: sigma=1.d0 
real*8, parameter :: sigma2=sigma**2
integer iat, jat
do iat=1,nat-1
do jat=iat+1,nat
    dx=xyz(1,iat)-xyz(1,jat)
    dy=xyz(2,iat)-xyz(2,jat)
    dz=xyz(3,iat)-xyz(3,jat)
    r2=dx*dx+dy*dy+dz*dz
    t2=sigma2/r2
    t6=t2*t2*t2
    epot =epot + 4.d0*eps*(t6-1.d0)*t6
    ff= 48.d0*eps*(t6-0.5d0)*t6/r2
    fxyz(1,iat)=fxyz(1,iat) +ff*dx
    fxyz(1,jat)=fxyz(1,jat) -ff*dx
    fxyz(2,iat)=fxyz(2,iat) +ff*dy
    fxyz(2,jat)=fxyz(2,jat) -ff*dy
    fxyz(3,iat)=fxyz(3,iat) +ff*dz
    fxyz(3,jat)=fxyz(3,jat) -ff*dz
enddo
enddo
endsubroutine

You might want to show us what you have tried so far. Do you understand the Fortran syntax? — rtoijala
– rtoijala, Commented Jun 7, 2019 at 6:42
You could use Fortran :-) Joke aside, this type of "loop intensive" code will perform poorly in Python. — Pierre de Buyl
– Pierre de Buyl, Commented Jun 7, 2019 at 11:22

Jamil KR · Accepted Answer · 2019-06-07 09:09:15Z

You are able to use fortran subroutines and functions in python. I recommend you use f2py from numpy. In the next link you will find the documentation: https://docs.scipy.org/doc/numpy/f2py/

If you want to use a fortran subroutine in python, you should be careful because the subroutine will be transformed to a python function. You must write the intent of the variables in your program. For example, you have the a module test in the file file.f90:

module test
  implicit none
  contains
  subroutine sub(a,b)
    real, intent(in):: a
    real, intent(out):: b
    b=2.0*a
  end subroutine sub
end module test

Now you can create the python module using f2py (command line):

f2py -c -m python_mod file.f90

f2py will have created a file called python_mod.so in the working folder. Now you can use it in a python program as a function :

from python_mod import test

c = test.sub(3.0)
print(c)

The output will be 6.0.

I hope you find it useful. Good luck!

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