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[mbluo@yahoo.com.cn]

Hi, Abinit members,

I want to calculate the shear modulus of Si as an example. The input file is 
attached. By varying the angle, I get energy - angle relation and from which 
the shear modulus is determined. However, I find the shear modulus (about 104 
GPa) is bigger than the bulk modulus (about 94 GPa). This is contrary to the 
experimental results. LDA and GGA pseudopotentials give almost the same 
result.

Your help and commants are helpful. Thanks

Mengbo Luo

***************************

The Input file is:

# Crystalline silicon : computation of the total energy
#  ZnS structure

ndtset 6
jdtset 5 6 7 8 9 10
 
chkprim 0

#Definition of the unit cell  OPT:10.20360336
#  angdeg: angdeg(1) is the angle between the 2nd and 3rd vectors, 
#          angdeg(2) is the angle between the 1st and 3rd vectors, 
#          angdeg(3) is the angle between the 1st and 2nd vectors, 
# 1 1 0 

acell1  2*10.20360336   10.20360336
angdeg1 90   90   90
acell2  2*10.20360336   10.20515765494
angdeg2 90   89   90
acell3  2*10.20360336   10.20982290826
angdeg3 90   88   90
acell4  2*10.20360336   10.21760623424
angdeg4 90   87   90
acell5  2*10.20360336   10.22851951947
angdeg5 90   86   90
acell6  2*10.20360336   10.24257946719
angdeg6 90   85   90
acell7  2*10.20360336   10.25980765986
angdeg7 90   84   90
acell8  2*10.20360336   10.280230640285
angdeg8 90   83   90
acell9  2*10.20360336   10.30388001197 
angdeg9 90   82   90
acell10  2*10.20360336  10.33079255937
angdeg10 90   81   90


#Definition of the k-point grid
kptopt 1         
nshiftk 1    
shiftk 0.5 0.5 0.5
                 # In cartesian coordinates, this grid is simple cubic, and
                  # actually corresponds to the
                  # so-called 4x4x4 Monkhorst-Pack grid

ngkpt  3*8

getwfk -1    

#Definition of the atom types
ntype 1  
zatnum 14
                         

#Definition of the atoms
natom 8          # There are two atoms
type 8*1         # They both are of type 1, that is, Silicon.
xred
   0.0  0.0  0.0  
   0.5  0.5  0.0
   0.0  0.5  0.5
   0.5  0.0  0.5
   0.25  0.25  0.25
   0.75  0.75  0.25
   0.25  0.75  0.75
   0.75  0.25  0.75

#Definition of the planewave basis set
ecut  40.0       

#Definition of the SCF procedure
nstep 15         
toldfe 1.0d-6     
diemac 12.0