anchor.diff:             call dimer_io%image%append(colloids%image)
           end if
   
  -        com_pos = ( colloids%pos(:,1)+colloids%image(:,1)*solvent_cells%edges + &
  +        call vacf%add(i*N_MD_steps+j, correlate_block_dot, xvec=sum(colloids%vel, dim=2)/2)
  +
  +        com_pos = (colloids%pos(:,1)+colloids%image(:,1)*solvent_cells%edges + &
                colloids%pos(:,2)+colloids%image(:,2)*solvent_cells%edges)
   
           unit_r = rel_pos(colloids%pos(:,1), colloids%pos(:,2), solvent_cells%edges)
  
positive.code:   call msd%init(block_length, n_blocks, dim=3)
    call oacf%init(block_length, n_blocks, dim=3)
    call omega_acf%init(block_length, n_blocks)
  
    if (dimer) then
       colloids% species(1) = 1
  
negative.code:   call h5md_write_dataset(group, 'count', omega_acf%count)
    call h5md_write_dataset(group, 'step', colloid_sampling)
    call h5md_write_dataset(group, 'time', colloid_sampling*dt)
    call h5gclose_f(group, error)
  
    call h5gcreate_f(correlator_group, 'velocity_autocorrelation', group, error)
    call h5md_write_dataset(group, 'value', vacf%correlation)
    call h5md_write_dataset(group, 'count', vacf%count)
    call h5md_write_dataset(group, 'step', 1)
    call h5md_write_dataset(group, 'time', dt)
    call h5gclose_f(group, error)
  
    call h5gclose_f(correlator_group, error)
  