[Pw_forum] The overestimation of graphite lattice using vc-relax+vdW-DF

[WANG Wei]

Hello, everyone. As a benchmark test, I performed the vc-relax+vdW-DF
calculations for graphite. The opitimized lattice constants seem to be
unreasonable using vc-relax+vdW-DF. It is overestimated about about 29% for
a. I don't know what the problem is.

#############################################################################################
          total   stress  (Ry/bohr**3)                   (kbar)     P=
-0.01
  -0.00000002   0.00000000   0.00000000          0.00      0.00      0.00
   0.00000000  -0.00000002   0.00000000          0.00      0.00      0.00
   0.00000000   0.00000000  -0.00000007          0.00      0.00     -0.01

     The maximum number of steps has been reached.
     End of BFGS Geometry Optimization
Begin final coordinates
     new unit-cell volume =    450.28054 a.u.^3 (    66.72469 Ang^3 )
CELL_PARAMETERS (alat=  4.68000000)
   1.292794235  -0.000000090   0.000000000
  -0.646397196   1.119592604   0.000000000
   0.000000000   0.000000000   3.034982637
ATOMIC_POSITIONS (crystal)
C       -0.000001693   0.000001693   0.250000000
C        0.000001693  -0.000001693   0.750000000
C        0.333331648   0.666668352   0.250000000
C        0.666668352   0.333331648   0.750000000
End final coordinates
######################################################################################



&control
   calculation = 'vc-relax'
!   restart_mode='from_scratch',
   prefix='graphite',
   tstress = .true.
   tprnfor = .true.
    pseudo_dir   = "/home/wangvei/works/quantum-espresso/pseudo",
    outdir       = "./",
    disk_io  = 'none'
    forc_conv_thr = 1.0e-3
   nstep =  45  ,
               etot_conv_thr = 1.0E-4 ,
               forc_conv_thr = 1.0D-3 ,
                      iprint = 1 ,
                 max_seconds = 36000 ,
                          dt = 100 ,
/
&system
    ibrav           = 4
    celldm(1)       = 4.68
    celldm(3)       = 2.70
    nat             = 4
    ntyp            = 1
    occupations = "smearing",
    smearing    = "gaussian",
    degauss     = 0.002D0,
    ecutwfc         = 40.0
    ecutrho         = 200
    input_dft       = 'vdw1'
/
&electrons
    conv_thr        = 1.0d-8
/
&ions
/
&cell
    press_conv_thr  = 0.0D0
    press           = 0.D0
    cell_dynamics   = 'bfgs'
!    cell_dofree     = 'z'
/
ATOMIC_SPECIES
C   12.00   C.pbe-paw_kj.UPF
ATOMIC_POSITIONS {crystal}
 C     0.00000   0.00000   0.25000
 C     0.00000   0.00000   0.75000
 C     0.33333   0.66667   0.25000
 C     0.66667   0.33333   0.75000
K_POINTS automatic
9 9 4 0 0 0
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