# [Pw_forum] cp.x and the autocorrelation function of dipoles

```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

*********    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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