**Calcualtion of e-ph coupling**

Here we describe how to calculate the vibrational modes of molecules using dynamical matrix. Note, the calculation of the e-ph coupling is meant only for non periodic systems or with only Gamma k-point.
  
iephc = 1

The calculation e-ph coupling is toggled by the keywords ''iephc'' in section ''&OPTION&'' of the input file ''fireball in''.

=== fireball.in: ===

<code>
&OPTION
basisfile = answer.bas 
icluster = 1
sigmatol = 0.00000001
max_scf_iterations = 100
iephc = 1
&END
</code>

Control parameters of the e-ph coupling calculation are set in file ''ephc.optional'', which must be present in the work directory. 
=== ephc.optional:  ===

<code>
ephc.dat                                                ! output file
eigens.dat                                              ! input file with reference eigenvalues (for zero displacement)
20                                                      ! effective temperature [K]
28.08                                                   ! effective mass [a.u.]
4                                                       ! number of eigenstates which will be analyzed
40,41,42,43                                             ! list of eigenstates (those listed in eigens.dat)
4                                                       ! number of modes to be analyzed (0...means all)
35,36,37,38                                             ! list of vibrational modes                     
</code>

Namely we have to define, how many and which molecular orbitals will be analyzed. Eigenvalues of the selected molecular orbitals must be listed in an input file, which name is defined on 2nd line ( ''eigens.dat''). This file must be present in the working directory and it has to have following structure:

<code>
-4.66010
-3.42451
-2.48913
-1.41136
</code>
where each line contains the corresponding eigenvalue of the molecular orbital. Optionally, we can also calculate only selected e-ph couplings, see lines  7 and 8 in the file ''ephc.optional''.