[CPMD-list] About geometry optimization

[naivebamboo Huang]

Juerg,
             Thanks for your help.
            The cut-off radius of the electrostatic interaction is estimated 
using a way related to the Wolf method, which indicates that the effects of 
electrostatic interaction in real space will be negligible beyond that 
cut-off due to the surrounding charged atoms.
           I know the dynamical matrix is not the actual interatonic force 
constant in pair bond model. But if we have created a bond model, is it 
possible to map the dynamic matrix into the bond force constant? Actually, 
we can get the dynamical matrix if the pair bond model has been created.

Thanks

Baoling


>From: Juerg Hutter <hutter at pci.unizh.ch>
>To: naivebamboo Huang <naivebamboo at hotmail.com>
>CC: cpmd-list at cpmd.org
>Subject: Re: [CPMD-list] About geometry optimization
>Date: Wed, 21 Mar 2007 18:34:38 +0100 (MET)
>
>Hi
>
>>Dear CPMD users,
>>     I am a novice to CPMD. I am trying to optimize the geometry of Bi2Te3 
>>and have some questions for the geometry optimization:
>>1. My understanding for the optimization of geometry is that if only 
>>"optimize geometry"  is used, the symmetry and the lattice constant of the 
>>cell will not change and only the coordinates of atoms in the cell are 
>>changed. Is this correct?
>
>Yes
>
>>
>>2. If I want to do the global optimization for both the lattice constant 
>>and the coordinates of atoms, should I just add "STEEPEST DESCENT CELL" 
>>except "OPTIMIZE GEOMETRY"?
>
>Yes. However, the cell optimization in CPMD is not very good. If your cell
>has a simple geometry (1 or 2 independent parameters) you are better
>off doing the optimization by hand.
>In addition, don't forget about the coonstant cutoff versus constant number 
>of plane waves problem (see old posts about this problem or check
>the literature).
>
>>
>>3.The atoms in the cell are charged and the electrostatic interaction is 
>>important for Bi2Te3. So if I try the geometry optimization, since the 
>>cut-off radius of the electrostatic interaction is around 12 Angstrom, do 
>>I need to choose a large cell to include the effects of this long-range 
>>interactions? or I can just use the primitive cell.
>>
>
>Long range Coulomb interactions are calculated by Ewald sums, no cutoff
>is used. I don't know where you got the 12 Angstroms from, but if you
>refer to the TESR variable, that's only the real space part.
>Again, if this is not clear to you, check the literature, e.g. our
>review (see www.cpmd.org for a link) or the book by R. Martin or any
>other paper on the plane wave method.
>
>
>>4. I would like to get the force constants  for each two-body bond with 
>>the optimal geometry for later calculations. But the atoms in a primitive 
>>cell are always bonded with the atoms in another primitive cell. If I get 
>>the force constant matrix, does it correspond the bonded two atoms? or 
>>just correponds to the atoms in the same primitive cell?
>>
>
>You can calculate the dynamical matrix of your system (at the gamma point
>for the phonons) with CPMD. However, what you get are the force constants
>of the normal modes. To assume that those force constants can be used
>as bond parameters usually fails. Remember, this is DFT not a pair 
>potential method.
>
>regards
>
>Juerg Hutter
>
>
>>Thanks a lot
>>
>>Baoling Huang
>>
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