How to use the matscipy.checkpoint.CheckpointCalculator function in matscipy

# first do a couple of calculations
        np.random.seed(0)
        atoms.rattle()
        cp_calc_1 = CheckpointCalculator(calc, logger=screen)
        atoms.set_calculator(cp_calc_1)
        e11 = atoms.get_potential_energy()
        f11 = atoms.get_forces()
        atoms.rattle()
        e12 = atoms.get_potential_energy()
        f12 = atoms.get_forces()
        
        # then re-read them from checkpoint file
        atoms = orig_atoms
        np.random.seed(0)
        atoms.rattle()
        cp_calc_2 = CheckpointCalculator(calc, logger=screen)
        atoms.set_calculator(cp_calc_2)
        e21 = atoms.get_potential_energy()
        f21 = atoms.get_forces()
        atoms.rattle()
        e22 = atoms.get_potential_energy()
        f22 = atoms.get_forces()

        self.assertAlmostEqual(e11, e21)
        self.assertAlmostEqual(e12, e22)
        self.assertArrayAlmostEqual(f11, f21)
        self.assertArrayAlmostEqual(f12, f22)

def rattle_calc(self, atoms, calc):
        try:
            os.remove('checkpoints.db')
        except OSError:
            pass

        orig_atoms = atoms.copy()        
        
        # first do a couple of calculations
        np.random.seed(0)
        atoms.rattle()
        cp_calc_1 = CheckpointCalculator(calc, logger=screen)
        atoms.set_calculator(cp_calc_1)
        e11 = atoms.get_potential_energy()
        f11 = atoms.get_forces()
        atoms.rattle()
        e12 = atoms.get_potential_energy()
        f12 = atoms.get_forces()
        
        # then re-read them from checkpoint file
        atoms = orig_atoms
        np.random.seed(0)
        atoms.rattle()
        cp_calc_2 = CheckpointCalculator(calc, logger=screen)
        atoms.set_calculator(cp_calc_2)
        e21 = atoms.get_potential_energy()
        f21 = atoms.get_forces()
        atoms.rattle()

self.logger.pr('retrieved results for {0} from checkpoint'.format(properties))
            # save results in calculator for next time
            if isinstance(self.calculator, Calculator):
                if not hasattr(self.calculator, 'results'):
                    self.calculator.results = {}
                self.calculator.results.update(dict(zip(properties, results)))
        except NoCheckpoint:
            if isinstance(self.calculator, Calculator):
                self.logger.pr('doing calculation of {0} with new-style calculator interface'.format(properties))
                self.calculator.calculate(atoms, properties, system_changes)
                results = [self.calculator.results[prop] for prop in properties]
            else:
                self.logger.pr('doing calculation of {0} with old-style calculator interface'.format(properties))
                results = []
                for prop in properties:
                    method_name = CheckpointCalculator.property_to_method_name[prop]
                    method = getattr(self.calculator, method_name)
                    results.append(method(atoms))
            _calculator = atoms.get_calculator()
            try:
                atoms.set_calculator(self.calculator)
                self.checkpoint.save(atoms, *results)
            finally:
                atoms.set_calculator(_calculator)

        self.results = dict(zip(properties, results))