Get forces without calculation in prepare_structures using VASP

Dear developers:

I am trying to generate reference structures for FCP following the tutorial, with changing the calculator from EMT into Vasp, and importing a POSCAR from work directory. However, a strange issue was occured.

At first, when the current work directory contains only a POSCAR structure file. The command prepare_structures(structures,atom_lat,calc) would submit a Vasp calculation, but with an error: "No such file or directory: 'OUTCAR'".

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/publics/.local/lib/python3.7/site-packages/hiphive/utilities.py", line 141, in prepare_structures
    return [prepare_structure(s, atoms_ideal, calc) for s in structures]
  File "/home/publics/.local/lib/python3.7/site-packages/hiphive/utilities.py", line 141, in <listcomp>
    return [prepare_structure(s, atoms_ideal, calc) for s in structures]
  File "/home/publics/.local/lib/python3.7/site-packages/hiphive/utilities.py", line 79, in prepare_structure
    forces = atoms_tmp.get_forces()
  File "/home/publics/.local/lib/python3.7/site-packages/ase/atoms.py", line 788, in get_forces
    forces = self._calc.get_forces(self)
  File "/home/publics/.local/lib/python3.7/site-packages/ase/calculators/abc.py", line 23, in get_forces
    return self.get_property('forces', atoms)
  File "/home/publics/.local/lib/python3.7/site-packages/ase/calculators/calculator.py", line 737, in get_property
    self.calculate(atoms, [name], system_changes)
  File "/home/publics/.local/lib/python3.7/site-packages/ase/calculators/vasp/vasp.py", line 331, in calculate
    self.read_results()
  File "/home/publics/.local/lib/python3.7/site-packages/ase/calculators/vasp/vasp.py", line 551, in read_results
    outcar = self.load_file('OUTCAR')
  File "/home/publics/.local/lib/python3.7/site-packages/ase/calculators/vasp/vasp.py", line 654, in load_file
    with open(filename, 'r') as fd:
FileNotFoundError: [Errno 2] No such file or directory: 'OUTCAR'

Then, after finishing of the Vasp calculation, files "OUTCAR" and "vasprun.xml" are created. When I executed the command prepare_structures(structures,atom_lat,calc) again, the arrays "forces" and "displacements" in every structure in the list "structures" are created automatically. However, the calculation of rattled structures are still not finished.

Are these arrays reasonable for FCP generation? Where are these parameters from? Is there something wrong collecting forces and displacements following these steps? Is there a better way to generate FCP with Vasp? My structure file has nearly 200 atoms so it takes time to do a force calculation.

I am using the following environments: Anaconda3 with python 3.7

Package	Version
ase	3.22.1
h5py	3.6.0
hiphive	0.7.1
matplotlib	3.5.0
numpy	1.20.3
phonopy	2.12.0
scikit-learn	1.0.1
scipy	1.7.3
spglib	1.16.3

And my code until the structure preparation is:

 from ase.io import read,write 
 from ase.calculators.vasp import Vasp 
 from hiphive.structure_generation import generate_mc_rattled_structures 
 from hiphive.utilities import prepare_structures 

 n_structures = 3
 cell_size = 1 
 cell_size_2d = 1 
 rattle_std = 0.05 
 minimum_distance = 1.2 
 calc=Vasp()
 is2d = 0 
 
 structures_fname = 'rattled_structures.extxyz'
 atom_lat=[read('POSCAR').repeat((cell_size,cell_size,cell_size)),read('POSCAR').repeat((cell_size_2d,cell_size_2d,1))][is2d]

 structures = generate_mc_rattled_structures(atom_lat, n_structures, rattle_std, minimum_distance)
 structures = prepare_structures(structures, atoms_lat, calc)