Constrained optimization and MD with constraints

[fgygi]

This is a simple example of Qbox calculation with a bond length constraint where the bond length is varied and the geometry is optimized for each value of the constraint. The system is the Si₄ molecule described in the test/si4gs directory of the Qbox distribution.

Code: Select all

# Si4 geometry optimization with constraint
# Si4 ground state
set cell 20 0 0  0 20 0  0 0 20 
species silicon Si_VBC_LDA-1.0.xml
atom Si1 silicon  3.700  0.000  0.000
atom Si2 silicon  0.000  2.200  0.000
atom Si3 silicon -3.700  0.000  0.000
atom Si4 silicon  0.000 -2.200  0.000
set ecut 6
set wf_dyn PSDA
set ecutprec 2
randomize_wf
run 0 200
set atoms_dyn CG
run 50 5
# add constraint
constraint define distance dSi Si2 Si4 4.6
constraint enforce
run 20  5
constraint set dSi 4.8
constraint enforce
run 20  5
constraint set dSi 5.0
constraint enforce
run 20  5

The following is a simple example of MD simulation with constraint for the Si₄ molecule. It includes a first MD run with a distance constraint of 4.6 bohr between atoms "Si2" and "Si4", and then another MD run with a distance constraint of 4.8 bohr. The force acting on the constraint can be extracted from the <constraint> element in the output.

Code: Select all

# Si4 MD with constraint
set cell 20 0 0  0 20 0  0 0 20 
species silicon Si_VBC_LDA-1.0.xml
atom Si1 silicon  3.700  0.000  0.000
atom Si2 silicon  0.000  2.200  0.000
atom Si3 silicon -3.700  0.000  0.000
atom Si4 silicon  0.000 -2.200  0.000
set ecut 6
set wf_dyn PSDA
set ecutprec 2
# compute initial ground state
randomize_wf
set scf_tol 1.e-6
run -atomic_density 0 50 10
# optimize initial geometry
randomize_r 0.1
set atoms_dyn CG
run 50 5
# add constraint
constraint define distance dSi Si2 Si4 4.6
constraint enforce
# reoptimize geometry with constraint
run 20  5
# run MD with constraint
set atoms_dyn MD
randomize_v 300
set dt 40
run 500 5
# modify constraint and run more MD
constraint set dSi 4.8
constraint enforce
# recompute ground state
run 0 50
# continue MD
run 500 5