ABINIT Discussion Forums

hello,everyone!
in lectures, i know two methods are often adopted to caculate bulk modulus by Birch-Murnaghan equation:
one is increasing isotrpic static pressure on the unit cell (Bravais lattice) step by step, then get the relevant energy, finally using Energy-Pressure-type Birch-Murnaghan equation to obtain the bulk modulus.
the other is by changing the "acell' Parameters, get the relevant energy, through Energy-Volume-type Birch-Murnaghan equation to obtain the bulk modulus; to do this, it is easy to obtain the bulk modulus of cubic cell (Bravais lattice), for the change of "acell" is equal for the three directions; but i don't know how to get bulk modulus of orthogonal, tetragonal, hexagonal, and other types by using this method? can anybody give some detailed guidance?
thanks!
best whishs
lzl

relax your structure using the strtarget input variable, you can then relax in the presence of hydrostatic pressure and get P as a function of ucvol (the unit cell volume) and then you don't need to go through the Eq of state.

Dear Pro. Zwanziger
thanks! I see.

Dear lzl8181,

I want to caclulate the bulk module for a cubic zinc Blende crystal structure of the binary compound CdS by methods you entioned , i prefer the second one i.e by increasing the "acell", i tried firstly through the toturespon (elastic toturial 6) and i got alot of warnings, actually i don't know how to calcuate it so please can you explain for me how i do that step by step and in below is my input file :


Best regards...



ndtset 12 # Total number of datasets (3*4)
udtset 3 4 # Double loop for k-sample convergence study

nspden 2
nsppol 2
nspinor 1

# Set 1 : Initial self-consistent and lattice optimization run

getwfk?1 0
ionmov?1 2 # Broyden lattice optimization scheme
ntime?1 5 # Maximim lattice optimization steps
optcell?1 1 # Optimize cell volume only
strfact?1 100 # Test convergence of stresses (Hartree/bohr^3) by
# multiplying by this factor and applying force
# convergence test
tolmxf?1 1.0e-6 # Convergence limit for forces as above
tolvrs?1 1.0d-18 # Need excellent convergence of GS quantities for RF runs

# Set 2 : Additional iteration to print density just at converged acell

prtden?2 1 # Third dataset needs density
tolvrs?2 1.0d-18

# Set 3 : Converge unoccupied wave functions

getden?3 -1 # Use density from previout set
tolwfr?3 5.0d-19 # Only wave function convergence can be used with
# non-self-consistent calculation
tolwfr23 1.0d-30 # This is simply for a reason of portability of automatic tests
nstep23 25 # This is simply for a reason of portability of automatic tests
nstep33 35 # This is simply for a reason of portability of automatic tests

# Set 4 : response-function calculations for all needed perturbations

kptopt?4 2 # Time-reversal only for RF calculation
nqpt?4 1
qpt?4 0 0 0 # By symmetry, only need one direction
rfdir?4 1 0 0
rfstrs?4 3 # Need both unaxial and shear strains
tolvrs?4 1.0d-12 # Need reasonable convergence of 1st-order quantities

#Common input data

#Double loop data passing

getcell -1 # Start from optimized (datasets ?2-?4) or previously
# optimized (datasets ?1) acell
getwfk -1 # Use last set of wave functions (except datasets ?1)


#Lattice definition
acell 3*10.96 # Starting value
dilatmx 1.05 # Allow for optimization
rprim 0.0 0.5 0.5
0.5 0.0 0.5
0.5 0.5 0.0

#Definition of the atom types and atoms
ntypat 2
znucl 48 16
natom 2
typat 1 2

#Atomic position
xred 0.0 0.0 0.0
0.25 0.25 0.25

#Definition of the plane wave basis set
ecut 20.947118 # Maximum kinetic energy cutoff (Hartree)
ecutsm 0.5 # Smoothing energy needed for lattice parameter
# optimization. This will be retained for
# consistency throughout.

#Definition of the k-point grid - loop over 3 k-point densities

ngkpt1? 6 6 6
ngkpt2? 8 8 8
ngkpt3? 10 10 10

nshiftk 4 # Use one copy of grid only (default)
shiftk 0.0 0.0 0.5 # This gives the usual fcc Monkhorst-Pack grid
0.0 0.5 0.0
0.5 0.0 0.0
0.5 0.5 0.5

#Definition of occupation numbers and number of bands
nband 10 # With metallic occup
occopt 4 # Femi-function smearing
tsmear 0.02

#Definition of the self-consistency procedure
iscf 5 # Use conjugate-gradient SCF cycle (datasets 1 & 3)
nstep 50 # Maximum number of SCF iterations
# This might not be enough for the very demanding tolwfr?3 above,
# but was chosen for portability reasons.
# enforce calculation of forces at each SCF step
optforces 1


#%%<BEGIN TEST_INFO>
#%% [setup]
#%% executable = abinit
#%% [files]
#%% files_to_test =
#%% telast_6.out, tolnlines= 0, tolabs= 0.000e+00, tolrel= 0.000e+00, fld_options = -medium
#%% psp_files = 13al.pspnc, 33as.pspnc
#%% [paral_info]
#%% max_nprocs = 1
#%% [extra_info]
#%% author =
#%% keywords =
#%% description =
#%%<END TEST_INFO>

jzwanzig wrote:relax your structure using the strtarget input variable, you can then relax in the presence of hydrostatic pressure and get P as a function of ucvol (the unit cell volume) and then you don't need to go through the Eq of state.

Dear Jzwanzig,
please help me out. I am working on cubic system and i have the following output

"Scale of Primitive Cell (acell) [bohr]
8.80000000000000E+00 8.80000000000000E+00 8.80000000000000E+00
Real space primitive translations (rprimd) [bohr]
0.00000000000000E+00 4.40000000000000E+00 4.40000000000000E+00
4.40000000000000E+00 0.00000000000000E+00 4.40000000000000E+00
4.40000000000000E+00 4.40000000000000E+00 0.00000000000000E+00
Unitary Cell Volume (ucvol) [Bohr^3]= 1.70368000000000E+02
Angles (23,13,12)= [degrees]
6.00000000000000E+01 6.00000000000000E+01 6.00000000000000E+01
Lengths [Bohr]
6.22253967444162E+00 6.22253967444162E+00 6.22253967444162E+00
Stress tensor in cartesian coordinates (strten) [Ha/bohr^3]
-9.31428390291595E-04 0.00000000000000E+00 0.00000000000000E+00
0.00000000000000E+00 -9.31428390291594E-04 0.00000000000000E+00
0.00000000000000E+00 0.00000000000000E+00 -9.31428390291593E-04
Total energy (etotal) [Ha]= -1.18290446959563E+01


Cartesian components of stress tensor (hartree/bohr^3)
sigma(1 1)= -9.31428390E-04 sigma(3 2)= 0.00000000E+00
sigma(2 2)= -9.31428390E-04 sigma(3 1)= 0.00000000E+00
sigma(3 3)= -9.31428390E-04 sigma(2 1)= 0.00000000E+00

-Cartesian components of stress tensor (GPa) [Pressure= 2.7404E+01 GPa]
- sigma(1 1)= -2.74035647E+01 sigma(3 2)= 0.00000000E+00
- sigma(2 2)= -2.74035647E+01 sigma(3 1)= 0.00000000E+00
- sigma(3 3)= -2.74035647E+01 sigma(2 1)= 0.00000000E+00"

I don't know values to pick from this output to calculate bulk modulus and pressure derivative of bulk modulus.
Thanks

The unit cell volume is 1.70368E2 bohr^3 (see ucvol) and the pressure is 2.7404E1 GPa (look for the word Pressure).