[CPMD-list] question about Fourier transform program of DIPOLE autocorrelation

[Axel Kohlmeyer]

On Wed, 27 Feb 2008, Yiming Zhang wrote:

YZ> Hi Axel,

hi yiming,

YZ> I have a question about your program package for Fourier transform 
YZ> of DIPOLE autocorrelation. In main.f90, the third input is:

please direct CPMD related questions to the CPMD mailing list.
this will make it easier for me to have my mail pre-sorted and
so other people who may have the same question but do not dare
to ask get an answer, too. thanks.

YZ>   PRINT *,"give time difference in input file (in atomic units):"
YZ>   PRINT *,"[CAUTION: this is the MD time step times the sampling
YZ> frequency.]"
YZ>   READ (*,*) data_t
YZ>   wave_fac = 219474.0_dbl/data_t
YZ> 
YZ> I can't understand what this mean. If "MD time step" means TIMESTEP in CPMD,
YZ> what does "sampling frequency" mean? Does it mean "DIPOLE DYNAMICS SAMPLE"
YZ> value in CPMD?

it means exactly what it says. the value you have to provide is the
time difference between two entries in the file you want to run through
the program. so if you have a DIPOLE file and use the SAMPLE keyword
with dipole dynamics, e.g. at 5, and use a timestep value of 4.0 then
the number you have to enter is 20.0 since the entries in the DIPOLE
file are 20.0 a.u. apart.

YZ> In the first thought, I thought sampling frequency sigma is a variable of
YZ> autocorrelation function which will be outputted to IR v.s. sigma figure.

nope, you won't get an autocorrelation function, but the fourier
transform of it, i.e. a plot of the spectral density vs. the 
frequency (omega) in reciprocal centimeters, similar to regular
IR spectra.

YZ> 
YZ> Do you have a example (with input DIPOLE and output figure) to test the
YZ> program? I just want to make sure I correctly use the program.

there should be one in the reference outputs of the cpmd-tutorial
on the bochum webserver.

cheers,
   axel.

YZ> 
YZ> Thanks,
YZ> Yiming Zhang
YZ> 

-- 
=======================================================================
Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
   Center for Molecular Modeling   --   University of Pennsylvania
Department of Chemistry, 231 S.34th Street, Philadelphia, PA 19104-6323
tel: 1-215-898-1582,  fax: 1-215-573-6233,  office-tel: 1-215-898-5425
=======================================================================
If you make something idiot-proof, the universe creates a better idiot.