Dear all

I am currently trying to calculate the IR spectrum by taking the FFT of the 
Autocorrelation function obtained from a CPMD run in cp.x (QE 6.1).


I recently stumbled across a post by Giuseppe Mattioli 
(http://qe-forge.org/pipermail/pw_forum/2015-July/107471.html) indicating that 
applying the MP correction would allow to calculate the FFT of the AC of the 
dipole; hence my attempt. I obtain (for CO2 centered inside a large empty box):


charge density inside the Wigner-Seitz cell:   16.00000000


     reference position (R0):        12.57468453   15.78158723   13.05433235 
bohr


     Dipole moments (with respect to x0):

     Elect    0.2108   -0.3623    0.1151 au,      0.5358   -0.9209    0.2925 
Debye

     Ionic    0.0000    0.0000    0.0000 au,      0.0000    0.0000    0.0000 
Debye

     Total    0.2108   -0.3623    0.1151 au,      0.5358   -0.9209    0.2925 
Debye


     Electrons quadrupole moment       -139.68377611 a.u.

          Ions quadrupole moment        101.51494350 a.u.

         Total quadrupole moment        -38.16883261 a.u.


     *********    MAKOV-PAYNE CORRECTION    *********


     Makov-Payne correction     0.00000000 a.u. =  0.000 eV (1st order, 1/a0)

                                0.00001738 a.u. =  0.000 eV (2nd order, 1/a0^3)

                                0.00001738 a.u. =  0.000 eV (total)


     corrected Total energy =   -37.17673195 a.u.


Now a LAMMPS script (http://lammps.sandia.gov/threads/msg63251.html) tells me 
that I am supposed to do the following:


calculate the autocorrelation function of the net dipole moment. Compute the 
Fourier transform of the autocorrelation function to get the spectrum.


Now I wonder which quantity to "autocorrelate". Is it


"Total_Dipole(timestep)+R0(timestep)"


Any help is greatly appreciated, especially I wonder if cppp.x is of any help 
to me on my journey! Input is below (for the ion dynamics)


Best,

Chris


Christoph Wolf, Seoul National University, Dept. of Materials Science and 
Engineering


 &control
    calculation = 'cp'
    title='CO2  MD'
    prefix='co2',
    pseudo_dir = '/home/chris/quantum_espresso/pseudo/',
    outdir='./'
    restart_mode='reset_counters',
    nstep=8000, iprint=10, isave=10,
    dt=5,
    ndr=50, ndw=51
 /
&SYSTEM
  ibrav = 1
  A=15 !relaxed cell value! 5.91 A
  nat = 3
  ntyp = 2
  ecutwfc =80.0
  assume_isolated='mp'
/
&electrons
        electron_dynamics = 'verlet'
        emass=700
/
 &ions
        ion_dynamics = 'verlet'
        ion_temperature='nose'
        fnosep=70.0
        tempw=300.0
/
ATOMIC_SPECIES
 C 12.0107 C_ONCV_PBE-Chris.UPF
 O 15.999 O_ONCV_PBE-Chris.UPF
ATOMIC_POSITIONS angstrom
 C   7.19147   7.73724   7.21980
 O   6.24948   7.68725   5.68537
 O   6.69993   9.42553   7.92219

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