[CPMD-list] IR and RDF

[Francesco Filippone]

On Sat, 28 Sep 2002, WANG, YIXUAN  wrote:

Dear Yixuan,
I try to help you.

> Dear all,
> 
>     Two questions need your help.
> 
>     1) I failed to find the way to extract radial distribution function
> (rdf) from CPMD
> manual. Could you tell me how to do this?

There is no keyword for this. Radial distribution function is to be 
calculated from a statistics, for instance from a Molecular Dynamics run. 
In CPMD you can extract (in post processing!) all the relevant information 
from the TRAJECTORY file; refer to the manual for the keywords MOLECULAR 
DYNAMICS and TRAJECTORY SAMPLE. You can apply the algorithm given in 
textbooks on MD, like M.P. Allen and D.J. Tildesley, Computer simulation 
of liquids, Oxford Science Publications, 1987.

> 
>     2) I am trying to calculate IR for (salt+ solvent ) system in CPMD
> simulation. 
> Is it possible to do this with CPMD? If so, please describe it.

I am not directly involved in CPMD development, so I don't know if the 
following point are completely up to date. You can either;

a) perform a vibrational analysis (keyword VIBRATIONAL ANALYSIS; otherwise 
the keywords PHONONS, LANCZOS with the LINEAR RESPONSE calculations). In 
this way you have a harmonic spectrum of the system you are simulating. 
This does not mean IR bands. If you want to know whether the modes you 
obtain are IR active or not, you have to perform a group theoretical 
analysis on the eigenvectors. Procedures are described, in, e.g., Wilson 
Decius Cross, Molecular Vibrations, Dover reprint, 1980, F.A. Cotton, 
Chemical Applications of Group Theory, Wiley, 1990. If you want IR 
intensities you have to look deeper into Chap. 7 of Wilson DEcius Cross

b) Calculate the power spectrum from a MD calculation. In this way you get 
rid of harmonic approximation, but you need a very very very long run to 
have a sufficient accuracy in numerical data. You can calculate the 
velocity-velocity autocorrelation function using the information in the 
TRAJECTORY file and perform a FT transform on it. Refer to Allen-Tildesley 
for all details. 
Again, you do not get the IR bands, rather all the vibrational degrees of 
freedom of the system.

c) Perform a similar analysis on the dypole dynamics (keyword DIPOLE 
DYNAMICS in a MD simulation) that is recorded in the file DIPOLE. In this 
case you have the IR bands. Again you do not get the intensities.

Hope could be useful (and not wrong :-)))), ciao,
F.


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