Example: Phase Transition Due To External Nonuniform Stress¶

#!/bin/sh

# Starting from hexagonal graphite we optimize the crystal
# under the influence of a non-uniform stress tensor.
# The system will undergo a phase transition to a diamond structure
AMS_JOBNAME=Graphite2Diamond $AMSBIN/ams << eor

Task GeometryOptimization
EngineAddons
    ExternalStress
        StressTensorVoigt 0 0 0.01 0 -0.0003 0
    End
End
System
    Atoms
        C  0.0000   0.0000   1.6507
        C  0.0000   0.0000  -1.6507
        C  0.0000  -1.4225   1.6507
        C  0.0000   1.4225  -1.6507
    End
    Lattice
        1.2319  -2.1338   0.0000
        1.2319   2.1338   0.0000
        0.0000   0.0000   6.6029
    End
End
GeometryOptimization
    MaxIterations 300
    OptimizeLattice Yes
    Convergence
        Energy 1.0e-6
        Gradients 1.0e-5
        Step 1.0e-4
    End
End
UseSymmetry No
Engine DFTB
    Model SCC-DFTB
    ResourcesDir DFTB.org/3ob-3-1
    DispersionCorrection D3-BJ
    KSpace
        Quality Basic
    End
EndEngine

eor

# The diamond structure is still compressed under the influence of the external stress.
# We now disable this term and relax the unit cell with the same DFTB model
AMS_JOBNAME=DiamondRelaxation $AMSBIN/ams << eor

Task GeometryOptimization
LoadSystem 
    File Graphite2Diamond.results/ams.rkf
End
GeometryOptimization
    MaxIterations 300
    OptimizeLattice Yes
    Convergence
        Energy 1.0e-6
        Gradients 1.0e-5
        Step 1.0e-4
    End
End
UseSymmetry No
LoadEngine Graphite2Diamond.results/dftb.rkf

eor