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Errors in importing Hellmann-Feynman forces file HFFILE. Simple errors are: (i) not commented comment line by ! or #, (ii) appearence of comma instead of point at numbers, (iii) too short "Name" of particles. Name is given in the second column of HFFILE, and it is a variable of CHARACTER*4, so one should leave four places for its format in HFFILE. PHONON versions 4.28 and earlier used only single character, thus such HFFILE should be corrected to four characters (including spaces). HFFILE calculated on Linux platform should be changed to Dos format before transporting it to Windows platform. The command is: todos HFFILE (Command transforming from Dos to Linux is fromdos FILE). After structure optimization the lattice constants and atomic positions initially introduced, may slightly differ from corresponding positions after optimization of the supercell. The HFFILE contains the finale optimized fractional atomic positions. To allow inspite of these differences to identify atoms in Phonon and in HFFILE one may increase to Tolerance of Positions at dialog Hellmann-Feynman/Supercell for HF Forces, or perform the procedure of updating the initial lattice constants and atomic position as described in the Manual. The PHONON may not identify correctly the atomic positions in the HFFILE, and this leads to error message. Example is that an atomic coordinate of 0.000010 (in fractional units) is interpreted by PHONON as 0.000000. If symmetry elements are involved with 0.000010 value, the number of calculated atoms may not match the number of atoms in atomic position file. These errors can be avoided by increasing the fractional coordinates in Position of Atoms, and in HFFILE to a larger deviation of respective atomic position (for example 0.000010 -> 0.000100). Note that slight changes of atomic positions influence only very little the phonon frequencies and/or polarization vectors.
A similar effect may occur, if a space group
of lower symmetry (subgroup) describes in fact a structure of the crystal
of higher symmetry space group, since deviations of atomic positions from high-symmetry sites are small (say 0.000010). The deviations must be interpreted by the symmetry elements, which in turn
perform this process with some tolerance. If the deviations are less then the tolerances,
then the trouble arrises.
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