anchor.diff:  
     end do
     call main%tac()
  +  write(*,*) ''
   
     write(*,*) 'n extra sorting', n_extra_sorting
     
  
positive.code:   solvent% force_old = solvent% force
    colloids% force_old = colloids% force
  
    write(*,*) ''
    write(*,*) '    i           |    e co so     |   e co co     |   kin co      |   kin so      |   total       |   temp        |'
    write(*,*) ''
  
    kin_co = (mass(1)*sum(colloids% vel(:,1)**2)+mass(2)*sum(colloids% vel(:,2)**2))/2
    call thermo_write
  
    call main%tic()
    do i = 1, N_loop
       md_loop: do j = 1, N_MD_steps
          call md_pos(solvent, dt)
  
  
negative.code: 
    mass(1) = rho * sigma_C**3 * 4 * 3.14159265/3
    mass(2) = rho * sigma_N**3 * 4 * 3.14159265/3
  
    call solvent% init(N,2) !there will be 2 species of solvent particles
  
  