# How to change the spin-orbit coupling strength in VASP?

Since the effect of spin-orbit coupling plays an important role in a topological insulator, How to change the spin-orbit coupling strength in VASP?

• Welcome to our community!
– Jack
Apr 5, 2021 at 8:04

How to change the spin-orbit coupling strength in VASP?

Firstly, one can't realize this just by changing the input parameters in the INCAR for VASP. To tune the strength of spin-orbit coupling in VASP, you need to modify the source file vasp_source_code_path/src/relativistic.F and recompile it. For example, if you want to reduce the strength to half you can just multiply 0.5d0 to the $$L \cdot S$$ term in line 129 of relativistic.F:

       DO I=0,1
DO J=0,1
DO M =1,2*LL+1
DO MP=1,2*LL+1
DLLMM(LMP+MP-1,LM+M-1,J+2*I+1)=DLLMM(LMP+MP-1,LM+M-1,J+2*I+1)+ &
0.5d0*SUM*LS(M,MP,I+2*J+1,LL)   !!!line 129 relativistic.F file
END DO
END DO
END DO
END DO


Note that you need to use the recompiled $$vasp_ncl$$ to run your calculation. As a benchmark, if you multiply 0.0d0 to the $$L \cdot S$$ term in line 129 of relativistic.F then you will receive the same result as the result obtained with vasp_std.

PS1: The answer is for the vasp.5.4.4.

PS2: You may take a look at this paper about the implementation of spin-orbit coupling in VASP. Or you may take a look at this post in matter modeling: Regarding spin-orbit coupling on DFT codes.

Hope it helps.

• Thacks for your answer. I will try to check it. Apr 5, 2021 at 9:02
• Is this legal? I mean, from the physical point of view...
– Camps
Apr 5, 2021 at 11:50
• @Camps Mathematically meaningful. The SOC effect of real materials is hardly changed. Maybe you can only replace the atoms to realize that.
– Jack
Apr 5, 2021 at 11:58
• My concern is that, using that hack, you can fake the results.
– Camps
Apr 5, 2021 at 12:11
• @Camps I guess the method is generally adopted just to investigate the gradual behavior of SOC.
– Jack
Apr 5, 2021 at 12:17