# How to start with biaxial tensile strain and compressive strain in TMDCs using VASP?

Good morning!

I would like to test the biaxial tensile strain and compressive strain in a hexagonal material from the TMDC group (for example HfS2). How to get it into VASP? What are the best ways to optimize geometry? Can I fall into some traps?

HfS2
1.0
3.6389749050         0.0000000000         0.0000000000
-1.8194881106         3.1514448821         0.0000000000
0.0000000000         0.0000000000        22.8899211884
Hf    S
1    2
Direct
0.000000000         0.000000000         0.500000000
0.666666985         0.333332986         0.563098013
0.333332986         0.666666985         0.436901987


Could you help me with this as example?

• +1. Thank you Milosz for contributing your very nice question here. Could you please put the contents of your screenshot, into a code block instead of doing a screenshot? What you have done is not the right way to present your question here. Sep 1 '20 at 18:58

How to get it into VASP?

The key point is to generate different POSCAR files. Once you prepare input files, you can perform the calculation with VASP. I will assume you are considering monolayer T-phase HfS2 and show how to generate compressive and tensile structures.

• Initial structure:[HfS2.vasp]

HfS2
1.0
3.6389749050         0.0000000000         0.0000000000
-1.8194881106         3.1514448821         0.0000000000
0.0000000000         0.0000000000        22.8899211884
Hf    S
1    2
Direct
0.000000000         0.000000000         0.500000000
0.666666985         0.333332986         0.563098013
0.333332986         0.666666985         0.436901987

• Then you can apply the biaxial strain for this structure by changing the length of $$\vec{a}$$ and $$\vec{b}$$ at the same time. In detail, you can do this with the following python script:

import numpy as np

def bistrain(path1,path2,strain):

with open(path1,'r') as f1:

lattice = np.zeros((3,3))
for i in range(3):lattice[i,:]=list(map(float,lines[2+i].strip().split()))

lattice[0:2,0:2]=lattice[0:2,0:2]*strain

with open(path2+'strain_'+str(strain)+'.vasp','w') as f2:
f2.write(lines[0])
f2.write(lines[1])

for j in range(3):f2.write("%20.16f  %20.16f  %20.16f" %(lattice[j,0],lattice[j,1],lattice[j,2])+'\n')

for k in range(5,len(lines)):f2.write(lines[k])

#==================================================================
path1='./HfS2.vasp'
path2='./'
#strain=0.99   ## compressive strain
strain=1.01    ## tensile strain
bistrain(path1,path2,strain)


What are the best ways to optimize geometry?

When the POSCAR is prepared, you can relax the structure referring to this answer.

Can I fall into some traps?

For the initial structure, you should use fractional coordinates rather than Cartesian coordinates.

May it hopes.

Just to add to jack's answer. This is easily implemented in ASE.

Here is the same example which creates biaxial strain POSCARs from -5% to 5% strain in 1% increments.

from ase.io import read
from os import makedirs
import numpy as np

for strain in np.arange(0.95, 1.05, 0.01):