QuantumATK Forum
QuantumATK => General Questions and Answers => Topic started by: majeedqau on April 27, 2018, 10:52
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As you know that to construct 2D materials (like graphene,Silicene or MoS2 etc) we need to put a vacuum in the z-direction which simply means to enlarge the magnitude of C vector. So my question is that we should need to keep fix the "Fractional" or "Cartesian" coordinate while enlarging the C parameter for the Vacuum creation?
My second question is that: To apply the in-plane biaxial strain we simply change the lattice parameter a and b. So to change a and b, we need to keep fix the "Fractional" or " Cartesian" coordinate in VNL?
Your help and suggestions would be highly appreciated in this regard.
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Regarding #1, you should keep it in Cartesian.
Regarding #1, it actually does not matter much whether you use Cartesian or Fractional because you are supposed to do ion relaxation after applying macroscopic strain to the cell, i.e.,it means that you change cell shape and/or volume. But I would suggest keeping the coordinates in Fractional because in some cases, e.g., when pure hydrostatic stress is applied, the symmetry of the lattice does not change, and the atoms keep their positions (given n Fractional coordinates) with respect to the new lattice vectors of the strained cell. In fact, even if there is only some hydrostatic contribution to the total strain, it would be beneficial to keep the coordinates in Fractional, since the new atomic position will then be closer to the equilibrium ones in the new (strained) lattice.
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First of all thank you very much for your response and clarification provided.
What I have gained from your explanation is:
Regarding#1: For the vacuum creation, the Cartesian coordinate is better to keep fix.
Regarding#2: While in the case to apply strain (i.e to change the lattice parameters a and b), Fractional Coordinate would be better to keep fix.
Is it right? please comments.
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It is right, except that you should also relax ions for the case #2 whatever coordinates you use.