econst2.xm

一个很好的分子动力学程序

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#  Example: Calculate elastic constant C44 for cubic lattice          #
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#  METHOD                                                             #
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#      Elastic constants can generally be obtained by applying a      #
#      strain to the lattice and calculating the second               #
#      derviative of the energy.  With XMD, only strains which        #
#      conform to the repeating boundary conditions can be            #
#      applied - shears, for example, will not work.  Strain are      #
#      typically applied using the SCALE command to contract or       #
#      expand the lattice and box together.                           #
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#      This example shows how to obtain the shear elastic constant    #
#      C44.  The method used is the same as for the elastic constant  #
#      C', except that the lattice is oriented within the simulation  #
#      box as 110, 1-10, 001 (instead of 100, 010, 001 as for the     #
#      case of C').
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#  SHEAR CONSTANT C44                                                 #
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#      Here we subject the lattice to a dilation in one 110           #
#      direction and a contraction along the other, while             #
#      maintaining a constant volume.  We orient the fcc cubic        #
#      lattice within the simulation box so the the lattice 110       #
#      direction lies in the x direction, and the 1-10 lies in        #
#      the y direction.  The z direction is 001.                      #
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#      The dilation and contraction is implemented with the           #
#      scale command, such as                                         #
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#                       SCALE 1.01 1/1.01 1.00                        #
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#      which applies the dilation and contraction, but maintains      #
#      a constant volume.                                             #
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#      We apply this scale several times, writing the energy each     #
#      time, in order to get a sample of the energy as a function     #
#      of strain.  The shear constant is then given by the formula    #
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#                            2    2                                   #
#                          a0    d E                                  #
#                   C44 =  ----  ---                                  #
#                                    2                                #
#                          4 V   d a                                  #
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#     where a  is the lattice constant in either the long or short    #
#                 direction.  You can use either as long as you       #
#                 remain consistent during the course of the          #
#                 calculation.                                        #
#           a0 is the value of a (lattice constant) at equilibrium    #
#           V  is the volume per atom                                 #
#           E  is the energy per atom as a function of a              #
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#
#  Read in NiAl potential
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read ../nial.txt
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#  Set the size of the simulation box
#    NX,NY,NZ are the number of unit cells in the X, Y and Z directions
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calc NX=6
calc NY=6
calc NZ=4
#
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#  Create an FCC lattice with unit cell length of 1 angstrom
#  and with orientation  110 1-10 001.  This is done by
#  first creating a cubic BCC lattice, and then applying a
#  "Bain" strain to obtain an FCC lattice in the desired orientation.
#
box NX NY NZ
particle 2
1  0.25 0.25 0.25
1  0.75 0.75 0.75
dup  (NX-1)  1 0 0
dup  (NY-1)  0 1 0
dup  (NZ-1)  0 0 1
#
#  Apply "Bain" strain
#
scale 1/sqrt(2) 1/sqrt(2) 1
#
#   Scale lattice to unit cell length of Ni, 3.52 angstroms
#
calc  A0=3.52
scale A0

#  Save energy of original lattice for later comparison

write energy


#
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#  SHEAR MODULUS C44
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#
#  DEL   is amount to change lattice constant
#  AX0   is old lattice constant in X direction
#  AY0   is old lattice constant in Y direction
#  AX    is new lattice constant in X direction
#  AY    is new lattice constant in Y direction
#
calc  DEL=0.1
calc  AX0=A0
calc  AY0=A0
repeat 3
   #  Write energy for current lattice
   WRITE AX0
   write energy

   #  Scale X and Y directions separately
   calc  AX=AX0+DEL
   calc  AY=AY0-DEL
   scale AX/AX0 AY/AY0 1.0

   #  Save new lattice sizes
   calc  AX0 = AX
   calc  AY0 = AY
end

#  revert to equilibrium lattice size
calc AX=3.52
calc AY=3.52
scale AX/AX0 AY/AY0 1.0

#  Write energy to test that lattice is original one
write energy