Example: Boron nitride optimization under external stress¶

#!/bin/sh

# Some (fairly arbitrary) external stress in atomic units:
external_stress='0.001 0.0003 -0.0007 0.0005 0.0004 0.001'

# The external stress applied here will result in pretty large deformations.
# Since the external stress energy term becomes more and more approximate the 
# larger the deformation from the initial cell is, after the first optimization, 
# one should run a second geometry optimization starting from the converged 
# results of the first one.

# It is always good practice to validate the optimization under external stress
# by computing the stress tensor of the final optimized geometry: the stress 
# tensor at the optimized geometry should match the external stress applied during
# the optimization, but with reversed signs.

AMS_JOBNAME=first_go $AMSBIN/ams << eor

Task GeometryOptimization

System
    Atoms
        B 0.0 0.0 0.0
        N 0.905 0.905 0.905
    End
    Lattice
        0.0 1.81 1.81
        1.81 0.0 1.81
        1.81 1.81 0.0
    End
    SuperCell 2 2 2
End

GeometryOptimization
    OptimizeLattice Yes
    # The convergence threshold should be samller than the applied external stress:
    Convergence Gradients=1.0E-4
End

# We want the external stress to break the symmetry:
UseSymmetry No

EngineAddons
    ExternalStress
        StressTensorVoigt $external_stress
    End
End

Engine ForceField
EndEngine
eor

# The second geometry optimization starting from the results of the first one:

AMS_JOBNAME=second_go $AMSBIN/ams << eor

Task GeometryOptimization

LoadSystem 
    File first_go.results/ams.rkf
End

GeometryOptimization
    OptimizeLattice Yes
    Convergence Gradients=1.0E-5
End

UseSymmetry No

EngineAddons
    ExternalStress
        StressTensorVoigt $external_stress
    End
End

LoadEngine first_go.results/forcefield.rkf
eor

# In the final calculation we simply compute the stress tensor of the final 
# geometry, which should be the opposite of the external stress tensor applied 
# during the optimization.

AMS_JOBNAME=sp $AMSBIN/ams << eor

Task SinglePoint

LoadSystem 
    File second_go.results/ams.rkf
End 

UseSymmetry No

Properties StressTensor=Yes

LoadEngine first_go.results/forcefield.rkf
eor

echo "Applied external stress tensor (order: xx,yy,zz,yz,xz,xy):"
echo $external_stress
echo "Resulting final stress tensor:"
$AMSBIN/amsreport sp.results/forcefield.rkf -r "AMSResults%StressTensor#12.6f##3"