Optimization of unit cell


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Optimization of unit cell leads to lat_c parameter that is 3 ang smaller than expt.
expt: a= 5.071 b= 5.071 c= 16.825
opt: a= 5.062 b= 5.062 c= 13.529

I have obtained similar results using:

ngrid 32 32 128
ngrid 32 32 96
nrid: 48 48 96

How can I improve the accuracy of my calculation?

My input deck follows:
=======================================================
title "NaNO3 testing - using R-3c symmetry"
echo

start NaNO3

memory 4000 mb

permanent_dir ./perm
scratch_dir ./scratch

geometry
 system crystal 
lat_a 5.071
lat_b 5.071
lat_c 16.825
alpha 90.0
beta 90.0
gamma 120.0
end
symmetry R-3c
Na 0.00000 0.00000 0.00000
N 0.00000 0.00000 0.25000
O 0.2448 0.00000 0.25000
end

pspw
  SIMULATION_CELL
ngrid 32 32 128
END
ewald_rcut 3.0
ewald_ncut 8
energy_cutoff 56.0
end

driver
  clear
maxiter 51
end

set nwpw:cif_filename NaNO3

set includestress .true. # tell driver to optimize unit cell
set nwpw:zero_forces .true. # zeros the forces on the atoms


task pspw optimize