# H2 molecule : study of translational and rotational modes

 ndtset   3
 ecut      12.0
 ecutsm       1.0
 ixc          1
  ngkpt       2 2 2  !! Better to use this than the gamma point
 diemac       2
 nband        1

### First data set : geometry optimization

 kptopt1     1
 tolmxf1     1.0d-5
  ntime1    10
 toldff1     1.0d-6
 ionmov1     3

### Second data set : accurate wave function calculation ###

   kptopt2      1
   tolwfr2      1.0d-22
   getwfk2      -1
 getxcart2     -1

### Third data set : atomic displacement ###

  kptopt3      2
    nqpt3      1
     qpt3      0.0 0.0 0.0
  rfphon3      1
 rfatpol3      1  2
   rfdir3      1 1 1
  tolvrs3      1.0d-9
  getwfk3     -1
getxcart3     -2

### Structure parameters ###

 acell 3*14.0
 natom   2
 nstep  40
     xcart  7.2669124276E-01  0.0000000000E+00  0.0000000000E+00
           -7.2669124276E-01  0.0000000000E+00  0.0000000000E+00
 ntypat   1
 typat    1  1
 znucl  1.00

 pp_dirpath "$ABI_PSPDIR"
 pseudos "PseudosGTH_pwteter/01h.pspgth"

#%%<BEGIN TEST_INFO>
#%% [setup]
#%% executable = abinit
#%% [files]
#%% files_to_test = 
#%%   t80.out, tolnlines = 0, tolabs = 1.289e-10, tolrel = 3.000e-10, fld_options =  -medium
#%% [paral_info]
#%% max_nprocs = 1
#%% [extra_info]
#%% authors = Unknown
#%% keywords = NC, DFPT
#%% description = 
#%%   H2 molecule : examine the rotational freedom.
#%%   The present test produces the following 
#%%   vibrational frequencies (with degeneracies indicated):
#%%   56.89 i cm-1 (2)
#%%   0.41   cm-1 (2)
#%%   1.05   cm-1
#%%   3800    cm-1 
#%%   The large frequency corresponds to the stretching
#%%   mode, and has the right order of magnitude.
#%%   The frequencies close to 1 cm-1 corresponds
#%%   to translation modes, and are small enough
#%%   for usual applications.
#%%   The  56.89 i cm-1 mode corresponds to rotation of 
#%%   the H2 molecule. The magnitude of this
#%%   frequency might seem quite
#%%   large. Here are the results of tests made to understand
#%%   this phenomenon. First, note that  
#%%   ecut 12  acell 3*14
#%%   Increasing the value of ecut to 25 decreases
#%%   the magnitude of the frequency to 36.8 cm-1 .
#%%   However, in order to continue to make it smaller,
#%%   the cell size must be increased , and an oscillatory
#%%   behaviour is observed :
#%%   3*16   45.6 i cm-1
#%%   3*18   22.7   cm-1
#%%   3*20   19.1 i cm-1
#%%   3*22   15.7 i cm-1
#%%   3*24   13.7   cm-1
#%%   Many other tests have been set up. In particular,
#%%   it was observed that the frequency of the oscillatory
#%%   behaviour changes with the ecut, and also that 
#%%   using the Gamma point, instead of the 1/4 1/4 1/4 k point
#%%   (used in this test) degrades the convergence.
#%%   The overall picture is as follows.
#%%   There are different reasons for the translation
#%%   and rotation modes to acquire a non-zero frequency
#%%   when plane waves and supercells are used.
#%%   Still, as concerns translations, only the
#%%   existence of a discretization of the XC grid
#%%   is important. For rotations, supercell effects
#%%   are also present :
#%%   - alignement of dipole or quadrupoles
#%%   - interaction between tails of wavefunctions, accross cells
#%%   Since the convergence in supercell size is oscillatory, we infer
#%%   that the breaking of the rotational invariance is mostly 
#%%   due to interaction between wavefunction tails.
#%%   This will be checked by confining the system in a spherical
#%%   well, in a forthcoming test.
#%%<END TEST_INFO>