[Pw_forum] IR

[Stefano Baroni]

Hi.
Would you be so kind as to sign your posts, with your name and  
academic affiliation?
MD conserves energy. It is natural that the temperature fluctuates. I  
am not an expert in MD, but I would also say that in order to  
calculate dynamical properties (such as the time correlations needed  
to get the IR spectrum) it is not a good idea to rescale the  
temperature, as "ion_temperature = 'rescaling' " seems to imply.
Finally, it does not seem to me a great idea either to use MD to  
calculate the IR spectrum of MgO at such a low temperature. Even when  
when you'll have learnt all the tricks of MD, I would expect the  
dynamics to be very harmonic, and hence not ergodic, thus requiring  
long (and difficult to estimate) equilibration times.
Why not using standard lattice dynamics? (vibrational frequencies +  
effective charges, both calculated bu "phonon").
Stefano Baroni


On Oct 6, 2009, at 2:44 AM, shoutian sun wrote:

> Dear all,
> I want to calculate IR spectrum with cp MD (cp.x) at the temperature  
> of 100K(without elec-field).
> And I just make a change the input file of example30 as follows. But  
> in the ourput file
> the temperature value: tempp changed every step. Can anyone tell me  
> how can I
> calculate IR with cp MD. Thanks in advance.
>
> >&CONTROL
> >  calculation = 'cp',
> >  restart_mode = 'from_scratch',
>  > nstep  = 10000,
> >  iprint = 100,
> >  isave  = 100,
>  > tstress = .FALSE.,
>   >tprnfor = .TRUE.,
> >  tefield = .TRUE.,
> >  prefix = 'mgo',
> >  dt=5.,
> >  pseudo_dir = '/ds13/zfliu/stsun/espresso-4.1/pseudo/',
> >  outdir='/ds13/zfliu/stsun/espresso-4.1/examples/example30/results/ 
> tmp'
> >/
> >
> >&SYSTEM
> >  ibrav = 1,
> >  celldm(1) = 15.912,
> >  celldm(2) = 0.0,
> >  celldm(3) = 0.0,
> >  celldm(4) = 0.0,
>  > celldm(5) = 0.0,
>  > celldm(6) = 0.0,
> >  nbnd=128,
> >  nelec=256,
> >  nat  =64,
> >  ntyp =2,
> >  ecutwfc = 40,
> >  ecutrho = 250,
> >  nr1b= 30, nr2b = 30, nr3b = 30,
> >  nspin=1,
> >/
> >
> >&ELECTRONS
> >  emass = 250.d0,
> >  emass_cutoff = 3.d0,
> >  electron_dynamics='damp', ortho_max=30
> >  electron_damping=0.05
> >  startingwfc = 'random',
> >  ampre = 0.02,
> >  efield = 0.0
> >  epol = 3
> >/
> >
> >&IONS
> >  ion_dynamics = 'verlet',
> >  ion_temperature = 'rescaling',
> >  tempw = 100,
> >/
> >
> >&CELL
>  > cell_dynamics = 'none',
> >/
> >
> >ATOMIC_SPECIES
>  >O  1.0 O.pz-rrkjus.UPF
>  >Mg 1.0 Mg.pz-n-vbc.UPF
> >ATOMIC_POSITIONS (bohr)
> >Mg   0.066300000   0.066300000   0.066300000
> >...........
>
> 好玩贺卡等你发，邮箱贺卡全新上线！ 
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---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -  
Trieste
http://stefano.baroni.me [+39] 040 3787 406 (tel) -528 (fax) /  
stefanobaroni (skype)

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