Structural Optimization to find equilibrium Lattice parameters

[Steven]

Hi everyone,
                     I am very new to DFT, and  I want to calculate the equilibrium lattice constants  of a tetragonal crystal system. The experimental lattice parameters are a=5.92 (angstrom) and c=11.8(angstrom). So, for plotting the total energy Vs Lattice constant  graph, I  vary the value of 'a' by  a step of 0.02 Angstrom from 4.72 Angstrom to 6.12 Angstrom with a fixed  value of c. But i am getting an increasing curve which has a minimum at 4.72 Angstrom which is far different from the experimental value and other theoretical values. Atomic positions are correct and the space group too. Should c/a ratio needs to be fixed while changing the values of 'a'?Please help .

[Jess Wellendorff]

Yes, I think fixing the c/a ratio when varying a is a good idea.
BTW: If you are simply looking for the lowest-energy structure of the crystal, you should consider using the OptimizeGeometry method for minimizing forces and stress.

[Steven]

Thank You Jess