def cluster_size(xyz_path, solvent):
    """Determines number of solvent molecules in a xyz file.
    
    Parameters
    ----------
    xyz_path : :obj:`str`
        Path to xyz file of interest.
    solvent : :obj:`list`
        Specifies solvents to determine the number of atoms included in a 
        molecule.
    
    Returns
    -------
    int
        Number of solvent molecules in specified xyz file.
    """

    with open(xyz_path, 'r') as xyz_file:

        line = xyz_file.readline()

        while line:
           
            split_line = line.split(' ')
            
            # Grabs number of atoms in xyz file.
            if len(split_line) == 1 and split_line[0] != '\n':

                atom_num = int(split_line[0])

                if solvent[0] == 'water':
                    molecule_num = atom_num / 3
                    return molecule_num
                elif solvent[0] == 'acetonitrile' or solvent[0] == 'acn':
                    molecule_num = atom_num / 6
                    return molecule_num

            line = xyz_file.readline()