[gpaw-users] Full optimization in plane wave mode

[Jón Steinar Garðarsson Mýrdal]

Hi Janne

When you change the shape or size of your super cell then the grid sampling will change and this can lead to a change in energy.
Therefore it is important to run calculations where you perform cell optimization with extra small grid spacing.
I would try lowering the value of h you are using.

Mvh.
Jón Steinar Garðarsson Mýrdal
Ph.d Student
DTU Energy Conversion
-------------------------------------------
Technical University of Denmark
Department of Energy Conversion and Storage
Frederiksborgvej 399
Building 775
4000 Roskilde
Direct +45 46775818
jsmy at dtu.dk
http://www.dtu.dk

________________________________________
From: gpaw-users-bounces at listserv.fysik.dtu.dk [gpaw-users-bounces at listserv.fysik.dtu.dk] On Behalf Of Janne Hirvi [janne.hirvi at uef.fi]
Sent: Wednesday, November 07, 2012 2:01 PM
To: gpaw-users at listserv.fysik.dtu.dk
Subject: [gpaw-users] Full optimization in plane wave mode

Dear GPAW-users,

I tried to perform "full" optimization (in orthogonal cell) for silica structure using plane wave mode.

The script was something like this

-----
mixerown=Mixer(beta=0.1, nmaxold=5, weight=50.0)
calc=GPAW(mode=PW(400), xc='PBE', kpts=(4,4,4), mixer=mixerown, txt=name+'.txt')
Bulk.set_calculator(calc)

ucf=UnitCellFilter(Bulk,mask=[1,1,1,0,0,0])
relax=BFGS(ucf)
traj=PickleTrajectory(name+'.traj','w',Bulk)
relax.attach(traj)
relax.run(fmax=0.03)
----

The corresponding calculation converged (stress and forces are small), but the energies for each step

Zero Kelvin:   -188.134359 (Step 1)
Zero Kelvin:   -187.264322
Zero Kelvin:   -187.394858
Zero Kelvin:   -187.853520
Zero Kelvin:   -187.879200
Zero Kelvin:   -188.081472
Zero Kelvin:   -188.048312
Zero Kelvin:   -188.051650
Zero Kelvin:   -188.081487
Zero Kelvin:   -188.096408 (Step 10)
Zero Kelvin:   -188.091418
Zero Kelvin:   -188.113318
Zero Kelvin:   -188.122375
Zero Kelvin:   -188.072067
Zero Kelvin:   -188.123096
Zero Kelvin:   -188.125491
Zero Kelvin:   -188.097103
Zero Kelvin:   -188.125748
Zero Kelvin:   -188.125478
Zero Kelvin:   -188.130157 (Step 20)
Zero Kelvin:   -188.146106
Zero Kelvin:   -188.122699
Zero Kelvin:   -188.065573
Zero Kelvin:   -188.071948
Zero Kelvin:   -188.033935
Zero Kelvin:   -187.960727
Zero Kelvin:   -187.975459
Zero Kelvin:   -187.954870
Zero Kelvin:   -187.941968
Zero Kelvin:   -187.914349 (Step 30)
Zero Kelvin:   -187.912083
Zero Kelvin:   -187.911463
Zero Kelvin:   -187.909650 (Step 33)

behave quite strangely. The energy minimum has been reached around 20 steps (only slightly smaller than that
for the initial guess) and the final energy is quite notably higher.

Do you know if this is related to large changes in cell dimensions when using plane wave mode or what might
be the reason for this?

Best regards,

Janne Hirvi

-----
Janne Hirvi
PhD (Physical Chemistry), Researcher
University of Eastern Finland, Department of Chemistry
P.O.Box 111, 80101 Joensuu, FINLAND
Tel: +358 50 4422594 & +358 50 3474223




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