I have optimized the crystal structure with conp flag and it got converged. Later I have fixed the cell parameters a, b, c and optimized the crystal structure with conv flag. I got different energy values, which supposed to be the same.
Once the system got converged with constant pressure optimization if we fix the cell parameters and reoptimize with a constant volume flag we must get the same energy. However different values have been obtained in my case. Can you explain the reason behind this?