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# Source code for ase.io.vasp

    """
    This module contains functionality for reading and writing an ASE
    Atoms object in VASP POSCAR format.

    """

    import re

    import numpy as np

    from ase import Atoms
    from ase.utils import reader, writer
    from ase.io.utils import ImageIterator
    from ase.io import ParseError
    from .vasp_parsers import vasp_outcar_parsers as vop
    from pathlib import Path

    __all__ = [
        'read_vasp', 'read_vasp_out', 'iread_vasp_out', 'read_vasp_xdatcar',
        'read_vasp_xml', 'write_vasp', 'write_vasp_xdatcar'
    ]

    def get_atomtypes(fname):
        """Given a file name, get the atomic symbols.

        The function can get this information from OUTCAR and POTCAR
        format files.  The files can also be compressed with gzip or
        bzip2.

        """
        fpath = Path(fname)

        atomtypes = []
        atomtypes_alt = []
        if fpath.suffix == '.gz':
            import gzip
            opener = gzip.open
        elif fpath.suffix == '.bz2':
            import bz2
            opener = bz2.BZ2File
        else:
            opener = open
        with opener(fpath) as fd:
            for line in fd:
                if 'TITEL' in line:
                    atomtypes.append(line.split()[3].split('_')[0].split('.')[0])
                elif 'POTCAR:' in line:
                    atomtypes_alt.append(line.split()[2].split('_')[0].split('.')[0])

        if len(atomtypes) == 0 and len(atomtypes_alt) > 0:
            # old VASP doesn't echo TITEL, but all versions print out species lines
            # preceded by "POTCAR:", twice
            if len(atomtypes_alt) % 2 != 0:
                raise ParseError(f'Tried to get atom types from {len(atomtypes_alt)} "POTCAR": '
                                  'lines in OUTCAR, but expected an even number')
            atomtypes = atomtypes_alt[0:len(atomtypes_alt)//2]

        return atomtypes

    def atomtypes_outpot(posfname, numsyms):
        """Try to retrieve chemical symbols from OUTCAR or POTCAR

        If getting atomtypes from the first line in POSCAR/CONTCAR fails, it might
        be possible to find the data in OUTCAR or POTCAR, if these files exist.

        posfname -- The filename of the POSCAR/CONTCAR file we're trying to read

        numsyms -- The number of symbols we must find

        """
        posfpath = Path(posfname)

        # Check files with exactly same path except POTCAR/OUTCAR instead
        # of POSCAR/CONTCAR.
        fnames = [posfpath.with_name('POTCAR'),
                  posfpath.with_name('OUTCAR')]
        # Try the same but with compressed files
        fsc = []
        for fnpath in fnames:
            fsc.append(fnpath.parent / (fnpath.name + '.gz'))
            fsc.append(fnpath.parent / (fnpath.name + '.bz2'))
        for f in fsc:
            fnames.append(f)
        # Code used to try anything with POTCAR or OUTCAR in the name
        # but this is no longer supported

        tried = []
        for fn in fnames:
            if fn in posfpath.parent.iterdir():
                tried.append(fn)
                at = get_atomtypes(fn)
                if len(at) == numsyms:
                    return at

        raise ParseError('Could not determine chemical symbols. Tried files ' +
                         str(tried))

    def get_atomtypes_from_formula(formula):
        """Return atom types from chemical formula (optionally prepended
        with and underscore).
        """
        from ase.symbols import string2symbols
        symbols = string2symbols(formula.split('_')[0])
        atomtypes = [symbols[0]]
        for s in symbols[1:]:
            if s != atomtypes[-1]:
                atomtypes.append(s)
        return atomtypes

    @reader
    def read_vasp(filename='CONTCAR'):
        """Import POSCAR/CONTCAR type file.

        Reads unitcell, atom positions and constraints from the POSCAR/CONTCAR
        file and tries to read atom types from POSCAR/CONTCAR header, if this fails
        the atom types are read from OUTCAR or POTCAR file.
        """

        from ase.constraints import FixAtoms, FixScaled
        from ase.data import chemical_symbols

        fd = filename
        # The first line is in principle a comment line, however in VASP
        # 4.x a common convention is to have it contain the atom symbols,
        # eg. "Ag Ge" in the same order as later in the file (and POTCAR
        # for the full vasp run). In the VASP 5.x format this information
        # is found on the fifth line. Thus we save the first line and use
        # it in case we later detect that we're reading a VASP 4.x format
        # file.
        line1 = fd.readline()

        lattice_constant = float(fd.readline().split()[0])

        # Now the lattice vectors
        a = []
        for ii in range(3):
            s = fd.readline().split()
            floatvect = float(s[0]), float(s[1]), float(s[2])
            a.append(floatvect)

        basis_vectors = np.array(a) * lattice_constant

        # Number of atoms. Again this must be in the same order as
        # in the first line
        # or in the POTCAR or OUTCAR file
        atom_symbols = []
        numofatoms = fd.readline().split()
        # Check whether we have a VASP 4.x or 5.x format file. If the
        # format is 5.x, use the fifth line to provide information about
        # the atomic symbols.
        vasp5 = False
        try:
            int(numofatoms[0])
        except ValueError:
            vasp5 = True
            atomtypes = numofatoms
            numofatoms = fd.readline().split()

        # check for comments in numofatoms line and get rid of them if necessary
        commentcheck = np.array(['!' in s for s in numofatoms])
        if commentcheck.any():
            # only keep the elements up to the first including a '!':
            numofatoms = numofatoms[:np.arange(len(numofatoms))[commentcheck][0]]

        if not vasp5:
            # Split the comment line (first in the file) into words and
            # try to compose a list of chemical symbols
            from ase.formula import Formula
            atomtypes = []
            for word in line1.split():
                word_without_delims = re.sub(r"-|_|,|\.|=|[0-9]|^", "", word)
                if len(word_without_delims) < 1:
                    continue
                try:
                    atomtypes.extend(list(Formula(word_without_delims)))
                except ValueError:
                    # print(atomtype, e, 'is comment')
                    pass
            # Now the list of chemical symbols atomtypes must be formed.
            # For example: atomtypes = ['Pd', 'C', 'O']

            numsyms = len(numofatoms)
            if len(atomtypes) < numsyms:
                # First line in POSCAR/CONTCAR didn't contain enough symbols.

                # Sometimes the first line in POSCAR/CONTCAR is of the form
                # "CoP3_In-3.pos". Check for this case and extract atom types
                if len(atomtypes) == 1 and '_' in atomtypes[0]:
                    atomtypes = get_atomtypes_from_formula(atomtypes[0])
                else:
                    atomtypes = atomtypes_outpot(fd.name, numsyms)
            else:
                try:
                    for atype in atomtypes[:numsyms]:
                        if atype not in chemical_symbols:
                            raise KeyError
                except KeyError:
                    atomtypes = atomtypes_outpot(fd.name, numsyms)

        for i, num in enumerate(numofatoms):
            numofatoms[i] = int(num)
            [atom_symbols.append(atomtypes[i]) for na in range(numofatoms[i])]

        # Check if Selective dynamics is switched on
        sdyn = fd.readline()
        selective_dynamics = sdyn[0].lower() == 's'

        # Check if atom coordinates are cartesian or direct
        if selective_dynamics:
            ac_type = fd.readline()
        else:
            ac_type = sdyn
        cartesian = ac_type[0].lower() == 'c' or ac_type[0].lower() == 'k'
        tot_natoms = sum(numofatoms)
        atoms_pos = np.empty((tot_natoms, 3))
        if selective_dynamics:
            selective_flags = np.empty((tot_natoms, 3), dtype=bool)
        for atom in range(tot_natoms):
            ac = fd.readline().split()
            atoms_pos[atom] = (float(ac[0]), float(ac[1]), float(ac[2]))
            if selective_dynamics:
                curflag = []
                for flag in ac[3:6]:
                    curflag.append(flag == 'F')
                selective_flags[atom] = curflag
        if cartesian:
            atoms_pos *= lattice_constant
        atoms = Atoms(symbols=atom_symbols, cell=basis_vectors, pbc=True)
        if cartesian:
            atoms.set_positions(atoms_pos)
        else:
            atoms.set_scaled_positions(atoms_pos)
        if selective_dynamics:
            constraints = []
            indices = []
            for ind, sflags in enumerate(selective_flags):
                if sflags.any() and not sflags.all():
                    constraints.append(FixScaled(atoms.get_cell(), ind, sflags))
                elif sflags.all():
                    indices.append(ind)
            if indices:
                constraints.append(FixAtoms(indices))
            if constraints:
                atoms.set_constraint(constraints)
        return atoms

    def iread_vasp_out(filename, index=-1):
        """Import OUTCAR type file, as a generator."""
        it = ImageIterator(vop.outcarchunks)
        return it(filename, index=index)

    @reader
    def read_vasp_out(filename='OUTCAR', index=-1):
        """Import OUTCAR type file.

        Reads unitcell, atom positions, energies, and forces from the OUTCAR file
        and attempts to read constraints (if any) from CONTCAR/POSCAR, if present.
        """
        # "filename" is actually a file-descriptor thanks to @reader
        g = iread_vasp_out(filename, index=index)
        # Code borrowed from formats.py:read
        if isinstance(index, (slice, str)):
            # Return list of atoms
            return list(g)
        else:
            # Return single atoms object
            return next(g)

    @reader
    def read_vasp_xdatcar(filename='XDATCAR', index=-1):
        """Import XDATCAR file

           Reads all positions from the XDATCAR and returns a list of
           Atoms objects.  Useful for viewing optimizations runs
           from VASP5.x

           Constraints ARE NOT stored in the XDATCAR, and as such, Atoms
           objects retrieved from the XDATCAR will not have constraints set.
        """
        fd = filename  # @reader decorator ensures this is a file descriptor
        images = list()

        cell = np.eye(3)
        atomic_formula = str()

        while True:
            comment_line = fd.readline()
            if "Direct configuration=" not in comment_line:
                try:
                    lattice_constant = float(fd.readline())
                except Exception:
                    # XXX: When would this happen?
                    break

                xx = [float(x) for x in fd.readline().split()]
                yy = [float(y) for y in fd.readline().split()]
                zz = [float(z) for z in fd.readline().split()]
                cell = np.array([xx, yy, zz]) * lattice_constant

                symbols = fd.readline().split()
                numbers = [int(n) for n in fd.readline().split()]
                total = sum(numbers)

                atomic_formula = ''.join('{:s}{:d}'.format(sym, numbers[n])
                                         for n, sym in enumerate(symbols))

                fd.readline()

            coords = [
                np.array(fd.readline().split(), float) for ii in range(total)
            ]

            image = Atoms(atomic_formula, cell=cell, pbc=True)
            image.set_scaled_positions(np.array(coords))
            images.append(image)

        if not index:
            return images
        else:
            return images[index]

    def __get_xml_parameter(par):
        """An auxiliary function that enables convenient extraction of
        parameter values from a vasprun.xml file with proper type
        handling.

        """
        def to_bool(b):
            if b == 'T':
                return True
            else:
                return False

        to_type = {'int': int, 'logical': to_bool, 'string': str, 'float': float}

        text = par.text
        if text is None:
            text = ''

        # Float parameters do not have a 'type' attrib
        var_type = to_type[par.attrib.get('type', 'float')]

        try:
            if par.tag == 'v':
                return list(map(var_type, text.split()))
            else:
                return var_type(text.strip())
        except ValueError:
            # Vasp can sometimes write "*****" due to overflow
            return None

    def read_vasp_xml(filename='vasprun.xml', index=-1):
        """Parse vasprun.xml file.

        Reads unit cell, atom positions, energies, forces, and constraints
        from vasprun.xml file
        """

        import xml.etree.ElementTree as ET
        from ase.constraints import FixAtoms, FixScaled
        from ase.calculators.singlepoint import (SinglePointDFTCalculator,
                                                 SinglePointKPoint)
        from ase.units import GPa
        from collections import OrderedDict

        tree = ET.iterparse(filename, events=['start', 'end'])

        atoms_init = None
        calculation = []
        ibz_kpts = None
        kpt_weights = None
        parameters = OrderedDict()

        try:
            for event, elem in tree:

                if event == 'end':
                    if elem.tag == 'kpoints':
                        for subelem in elem.iter(tag='generation'):
                            kpts_params = OrderedDict()
                            parameters['kpoints_generation'] = kpts_params
                            for par in subelem.iter():
                                if par.tag in ['v', 'i']:
                                    parname = par.attrib['name'].lower()
                                    kpts_params[parname] = __get_xml_parameter(par)

                        kpts = elem.findall("varray[@name='kpointlist']/v")
                        ibz_kpts = np.zeros((len(kpts), 3))

                        for i, kpt in enumerate(kpts):
                            ibz_kpts[i] = [float(val) for val in kpt.text.split()]

                        kpt_weights = elem.findall('varray[@name="weights"]/v')
                        kpt_weights = [float(val.text) for val in kpt_weights]

                    elif elem.tag == 'parameters':
                        for par in elem.iter():
                            if par.tag in ['v', 'i']:
                                parname = par.attrib['name'].lower()
                                parameters[parname] = __get_xml_parameter(par)

                    elif elem.tag == 'atominfo':
                        species = []

                        for entry in elem.find("array[@name='atoms']/set"):
                            species.append(entry[0].text.strip())

                        natoms = len(species)

                    elif (elem.tag == 'structure'
                          and elem.attrib.get('name') == 'initialpos'):
                        cell_init = np.zeros((3, 3), dtype=float)

                        for i, v in enumerate(
                                elem.find("crystal/varray[@name='basis']")):
                            cell_init[i] = np.array(
                                [float(val) for val in v.text.split()])

                        scpos_init = np.zeros((natoms, 3), dtype=float)

                        for i, v in enumerate(
                                elem.find("varray[@name='positions']")):
                            scpos_init[i] = np.array(
                                [float(val) for val in v.text.split()])

                        constraints = []
                        fixed_indices = []

                        for i, entry in enumerate(
                                elem.findall("varray[@name='selective']/v")):
                            flags = (np.array(
                                entry.text.split() == np.array(['F', 'F', 'F'])))
                            if flags.all():
                                fixed_indices.append(i)
                            elif flags.any():
                                constraints.append(FixScaled(cell_init, i, flags))

                        if fixed_indices:
                            constraints.append(FixAtoms(fixed_indices))

                        atoms_init = Atoms(species,
                                           cell=cell_init,
                                           scaled_positions=scpos_init,
                                           constraint=constraints,
                                           pbc=True)

                    elif elem.tag == 'dipole':
                        dblock = elem.find('v[@name="dipole"]')
                        if dblock is not None:
                            dipole = np.array(
                                [float(val) for val in dblock.text.split()])

                elif event == 'start' and elem.tag == 'calculation':
                    calculation.append(elem)

        except ET.ParseError as parse_error:
            if atoms_init is None:
                raise parse_error
            if calculation and calculation[-1].find("energy") is None:
                calculation = calculation[:-1]
            if not calculation:
                yield atoms_init

        if calculation:
            if isinstance(index, int):
                steps = [calculation[index]]
            else:
                steps = calculation[index]
        else:
            steps = []

        for step in steps:
            # Workaround for VASP bug, e_0_energy contains the wrong value
            # in calculation/energy, but calculation/scstep/energy does not
            # include classical VDW corrections. So, first calculate
            # e_0_energy - e_fr_energy from calculation/scstep/energy, then
            # apply that correction to e_fr_energy from calculation/energy.
            lastscf = step.findall('scstep/energy')[-1]
            dipoles = step.findall('scstep/dipole')
            if dipoles:
                lastdipole = dipoles[-1]
            else:
                lastdipole = None

            de = (float(lastscf.find('i[@name="e_0_energy"]').text) -
                  float(lastscf.find('i[@name="e_fr_energy"]').text))

            free_energy = float(step.find('energy/i[@name="e_fr_energy"]').text)
            energy = free_energy + de

            cell = np.zeros((3, 3), dtype=float)
            for i, vector in enumerate(
                    step.find('structure/crystal/varray[@name="basis"]')):
                cell[i] = np.array([float(val) for val in vector.text.split()])

            scpos = np.zeros((natoms, 3), dtype=float)
            for i, vector in enumerate(
                    step.find('structure/varray[@name="positions"]')):
                scpos[i] = np.array([float(val) for val in vector.text.split()])

            forces = None
            fblocks = step.find('varray[@name="forces"]')
            if fblocks is not None:
                forces = np.zeros((natoms, 3), dtype=float)
                for i, vector in enumerate(fblocks):
                    forces[i] = np.array(
                        [float(val) for val in vector.text.split()])

            stress = None
            sblocks = step.find('varray[@name="stress"]')
            if sblocks is not None:
                stress = np.zeros((3, 3), dtype=float)
                for i, vector in enumerate(sblocks):
                    stress[i] = np.array(
                        [float(val) for val in vector.text.split()])
                stress *= -0.1 * GPa
                stress = stress.reshape(9)[[0, 4, 8, 5, 2, 1]]

            dipole = None
            if lastdipole is not None:
                dblock = lastdipole.find('v[@name="dipole"]')
                if dblock is not None:
                    dipole = np.zeros((1, 3), dtype=float)
                    dipole = np.array([float(val) for val in dblock.text.split()])

            dblock = step.find('dipole/v[@name="dipole"]')
            if dblock is not None:
                dipole = np.zeros((1, 3), dtype=float)
                dipole = np.array([float(val) for val in dblock.text.split()])

            efermi = step.find('dos/i[@name="efermi"]')
            if efermi is not None:
                efermi = float(efermi.text)

            kpoints = []
            for ikpt in range(1, len(ibz_kpts) + 1):
                kblocks = step.findall(
                    'eigenvalues/array/set/set/set[@comment="kpoint %d"]' % ikpt)
                if kblocks is not None:
                    for spin, kpoint in enumerate(kblocks):
                        eigenvals = kpoint.findall('r')
                        eps_n = np.zeros(len(eigenvals))
                        f_n = np.zeros(len(eigenvals))
                        for j, val in enumerate(eigenvals):
                            val = val.text.split()
                            eps_n[j] = float(val[0])
                            f_n[j] = float(val[1])
                        if len(kblocks) == 1:
                            f_n *= 2
                        kpoints.append(
                            SinglePointKPoint(kpt_weights[ikpt - 1], spin, ikpt,
                                              eps_n, f_n))
            if len(kpoints) == 0:
                kpoints = None

            atoms = atoms_init.copy()
            atoms.set_cell(cell)
            atoms.set_scaled_positions(scpos)
            atoms.calc = SinglePointDFTCalculator(atoms,
                                                  energy=energy,
                                                  forces=forces,
                                                  stress=stress,
                                                  free_energy=free_energy,
                                                  ibzkpts=ibz_kpts,
                                                  efermi=efermi,
                                                  dipole=dipole)
            atoms.calc.name = 'vasp'
            atoms.calc.kpts = kpoints
            atoms.calc.parameters = parameters
            yield atoms

    @writer
    def write_vasp_xdatcar(fd, images, label=None):
        """Write VASP MD trajectory (XDATCAR) file

        Only Vasp 5 format is supported (for consistency with read_vasp_xdatcar)

        Args:
            fd (str, fp): Output file
            images (iterable of Atoms): Atoms images to write. These must have
                consistent atom order and lattice vectors - this will not be
                checked.
            label (str): Text for first line of file. If empty, default to list of
                elements.

        """

        images = iter(images)
        image = next(images)

        if not isinstance(image, Atoms):
            raise TypeError("images should be a sequence of Atoms objects.")

        symbol_count = _symbol_count_from_symbols(image.get_chemical_symbols())

        if label is None:
            label = ' '.join([s for s, _ in symbol_count])
        fd.write(label + '\n')

        # Not using lattice constants, set it to 1
        fd.write('           1\n')

        # Lattice vectors; use first image
        float_string = '{:11.6f}'
        for row_i in range(3):
            fd.write('  ')
            fd.write(' '.join(float_string.format(x) for x in image.cell[row_i]))
            fd.write('\n')

        _write_symbol_count(fd, symbol_count)
        _write_xdatcar_config(fd, image, index=1)
        for i, image in enumerate(images):
            # Index is off by 2: 1-indexed file vs 0-indexed Python;
            # and we already wrote the first block.
            _write_xdatcar_config(fd, image, i + 2)

    def _write_xdatcar_config(fd, atoms, index):
        """Write a block of positions for XDATCAR file

        Args:
            fd (fd): writeable Python file descriptor
            atoms (ase.Atoms): Atoms to write
            index (int): configuration number written to block header

        """
        fd.write("Direct configuration={:6d}\n".format(index))
        float_string = '{:11.8f}'
        scaled_positions = atoms.get_scaled_positions()
        for row in scaled_positions:
            fd.write(' ')
            fd.write(' '.join([float_string.format(x) for x in row]))
            fd.write('\n')

    def _symbol_count_from_symbols(symbols):
        """Reduce list of chemical symbols into compact VASP notation

        args:
            symbols (iterable of str)

        returns:
            list of pairs [(el1, c1), (el2, c2), ...]
        """
        sc = []
        psym = symbols[0]
        count = 0
        for sym in symbols:
            if sym != psym:
                sc.append((psym, count))
                psym = sym
                count = 1
            else:
                count += 1
        sc.append((psym, count))
        return sc

    def _write_symbol_count(fd, sc, vasp5=True):
        """Write the symbols and numbers block for POSCAR or XDATCAR

        Args:
            f (fd): Descriptor for writable file
            sc (list of 2-tuple): list of paired elements and counts
            vasp5 (bool): if False, omit symbols and only write counts

        e.g. if sc is [(Sn, 4), (S, 6)] then write::

          Sn   S
           4   6

        """
        if vasp5:
            for sym, _ in sc:
                fd.write(' {:3s}'.format(sym))
            fd.write('\n')

        for _, count in sc:
            fd.write(' {:3d}'.format(count))
        fd.write('\n')

    @writer
    def write_vasp(filename,
                   atoms,
                   label=None,
                   direct=False,
                   sort=None,
                   symbol_count=None,
                   long_format=True,
                   vasp5=True,
                   ignore_constraints=False,
                   wrap=False):
        """Method to write VASP position (POSCAR/CONTCAR) files.

        Writes label, scalefactor, unitcell, # of various kinds of atoms,
        positions in cartesian or scaled coordinates (Direct), and constraints
        to file. Cartesian coordinates is default and default label is the
        atomic species, e.g. 'C N H Cu'.
        """

        from ase.constraints import FixAtoms, FixScaled, FixedPlane, FixedLine

        fd = filename  # @writer decorator ensures this arg is a file descriptor

        if isinstance(atoms, (list, tuple)):
            if len(atoms) > 1:
                raise RuntimeError('Don\'t know how to save more than ' +
                                   'one image to VASP input')
            else:
                atoms = atoms[0]

        # Check lattice vectors are finite
        if np.any(atoms.cell.cellpar() == 0.):
            raise RuntimeError(
                'Lattice vectors must be finite and not coincident. '
                'At least one lattice length or angle is zero.')

        # Write atom positions in scaled or cartesian coordinates
        if direct:
            coord = atoms.get_scaled_positions(wrap=wrap)
        else:
            coord = atoms.get_positions(wrap=wrap)

        constraints = atoms.constraints and not ignore_constraints

        if constraints:
            sflags = np.zeros((len(atoms), 3), dtype=bool)
            for constr in atoms.constraints:
                if isinstance(constr, FixScaled):
                    sflags[constr.a] = constr.mask
                elif isinstance(constr, FixAtoms):
                    sflags[constr.index] = [True, True, True]
                elif isinstance(constr, FixedPlane):
                    mask = np.all(np.abs(np.cross(constr.dir, atoms.cell)) < 1e-5,
                                  axis=1)
                    if sum(mask) != 1:
                        raise RuntimeError(
                            'VASP requires that the direction of FixedPlane '
                            'constraints is parallel with one of the cell axis')
                    sflags[constr.a] = mask
                elif isinstance(constr, FixedLine):
                    mask = np.all(np.abs(np.cross(constr.dir, atoms.cell)) < 1e-5,
                                  axis=1)
                    if sum(mask) != 1:
                        raise RuntimeError(
                            'VASP requires that the direction of FixedLine '
                            'constraints is parallel with one of the cell axis')
                    sflags[constr.a] = ~mask

        if sort:
            ind = np.argsort(atoms.get_chemical_symbols())
            symbols = np.array(atoms.get_chemical_symbols())[ind]
            coord = coord[ind]
            if constraints:
                sflags = sflags[ind]
        else:
            symbols = atoms.get_chemical_symbols()

        # Create a list sc of (symbol, count) pairs
        if symbol_count:
            sc = symbol_count
        else:
            sc = _symbol_count_from_symbols(symbols)

        # Create the label
        if label is None:
            label = ''
            for sym, c in sc:
                label += '%2s ' % sym
        fd.write(label + '\n')

        # Write unitcell in real coordinates and adapt to VASP convention
        # for unit cell
        # ase Atoms doesn't store the lattice constant separately, so always
        # write 1.0.
        fd.write('%19.16f\n' % 1.0)
        if long_format:
            latt_form = ' %21.16f'
        else:
            latt_form = ' %11.6f'
        for vec in atoms.get_cell():
            fd.write(' ')
            for el in vec:
                fd.write(latt_form % el)
            fd.write('\n')

        # Write out symbols (if VASP 5.x) and counts of atoms
        _write_symbol_count(fd, sc, vasp5=vasp5)

        if constraints:
            fd.write('Selective dynamics\n')

        if direct:
            fd.write('Direct\n')
        else:
            fd.write('Cartesian\n')

        if long_format:
            cform = ' %19.16f'
        else:
            cform = ' %9.6f'
        for iatom, atom in enumerate(coord):
            for dcoord in atom:
                fd.write(cform % dcoord)
            if constraints:
                for flag in sflags[iatom]:
                    if flag:
                        s = 'F'
                    else:
                        s = 'T'
                    fd.write('%4s' % s)
            fd.write('\n')