Keywords: user_guide

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## 4.1 Single-q calculation

The phonon code `ph.x` calculates normal modes at a given q-vector, starting from data files produced by `pw.x` with a simple SCF calculation. NOTE: the alternative procedure in which a band-structure calculation with `calculation='phonon'` was performed as an intermediate step is no longer implemented since version 4.1. It is also no longer needed to specify `lnscf=.true.` for ≠ 0.

The output data files appear in the directory specified by the variable `outdir`, with names specified by the variable `prefix`. After the output file(s) has been produced (do not remove any of the files, unless you know which are used and which are not), you can run `ph.x`.

The first input line of `ph.x` is a job identifier. At the second line the namelist `&INPUTPH` starts. The meaning of the variables in the namelist (most of them having a default value) is described in file `Doc/INPUT_PH.*`. Variables `outdir` and `prefix` must be the same as in the input data of `pw.x`. Presently you can specify `amass(i)` (a real variable) the atomic mass of atomic type i or you can use the default one deduced from the periodic table on the basis of the element name. If `amass(i)` is not given as input of `ph.x`, the one given as input in `pw.x` is used. When this is `0` the default one is used.

After the namelist you must specify the q-vector of the phonon mode, in Cartesian coordinates and in units of 2π/a.

Notice that the dynamical matrix calculated by `ph.x` at = 0 does not contain the non-analytic term occurring in polar materials, i.e. there is no LO-TO splitting in insulators. Moreover no Acoustic Sum Rule (ASR) is applied. In order to have the complete dynamical matrix at = 0 including the non-analytic terms, you need to calculate effective charges by specifying option `epsil=.true.` to `ph.x`. This is however not possible (because not physical!) for metals (i.e. any system subject to a broadening).

At = 0, use program `dynmat.x` to calculate the correct LO-TO splitting, IR cross sections, and to impose various forms of ASR. If `ph.x` was instructed to calculate Raman coefficients, `dynmat.x` will also calculate Raman cross sections for a typical experimental setup. Input documentation in the header of `PHonon/PH/dynmat.f90`.

See Example 01 for a simple phonon calculations in Si, Example 06 for fully-relativistic calculations (LDA) on Pt, Example 07 for fully-relativistic GGA calculations.

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