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Hi All

I am going to write a script to optimize the lattice parameter of a crystal.
For exemple:

Cubic BN
3.57
0.0 0.5 0.5
0.5 0.0 0.5
0.5 0.5 0.0
1 1
Direct
0.00 0.00 0.00
0.25 0.25 0.25


I want to fully optimize the unit cell including the shape and length of a,b,c. What troubled me is that I don't know how to transfer the lattice vectors in the POSCAR to lattice parameters (a,b,c). What is the arithmetic? Is it possible to control the angle of the cell as well as the lattice length?

Thanks in advance.

please use the automatic option to optimize the lattice (ISIF=3; PREC=accurate) , and do the optimization by hand at most after the automatic lattice optimization has converged. If you want the angles of the lattice to be changed as well (for this high-symmetry lattice type), you will have to switch off symmetrization (ISYM=0) and/or slightly disturb the cubic symmetry in the input geometry.
Please have a look at the online handbook (in the chapter about the POSCAR file) how the overall scaling factor of the lattice constant (2nd line) and the Bravais Matrix are used to give the atomic positions in Carthesian coordinates (if defined in direct coordinates).

Thanks a lot! I will try again.