# How can we make a mechanically unstable cubic system, stable

I have to calculate the elastic constants of a Cubic system using a Density functional theory-based software. A cubic system will be mechanically stable if it satisfies the Born-stability condition (eq. 6 from ref),

$$C_{11}−C_{12}>0 ;C_{11}+2C_{12}>0 ;C_{44}>0.\tag{1}$$

But in my case, the system failed to satisfy the first condition ($$C_{11}-C_{44}$$ should be > 0 but I am getting a negative value).

How I can make this system stable, like applying stress, strain or something else? My aim is to predict if this material is useful in photovoltaic cells (up to ~2eV) and in thermometric devices (up to 800 Kelvin).

• Did you do a full geometry optimization (relaxation) before calculating the elastic properties? – Camps Nov 26 '20 at 12:08
• Yes, I did the full geometry optimisation. – astha Nov 26 '20 at 13:23
• I gave you a +1 but please consider my edit, it's important. – Nike Dattani Nov 27 '20 at 4:38
• If your aim is to predict a material with a band gap of about 2 eV, why does it matter which specific structure it has? Can you not distort the crystal to remove the instability? It will change the symmetry of the system, but it may keep the 2 eV gap that is after all what you are really interested in... – ProfM Nov 27 '20 at 21:44