I have calculated band gap value of rutile TiO2 using Density Functional Theory with the help of Quantum ESPRESSO package.I got the band gap value around 5 eV. But from experimental reference, I could confirm that the band gap value is 3.2 eV. Then DFT will always underestimate the band gap value. But in my calculation it overestimate the band gap value? so I would like to correct the error. So kindly let me know what are the parameters have an impact on bandgap value.
In a DFT calculation this can happen if one uses an unreasonably large U values within DFT+U framework. Also I believe that hybrid functionals in their standard form (i.e. 25% of exact exchange) can overestimate the band gap for some materials. Has the pseudopotential you are using been tested carefully? Other factors I can think of is straining the lattcie parameter of the simulation cell.
Testing a pseudopotnetial is a nontrivial task and I would not get into it unless I'm the one who generated it. Usually one trusts the tests that the developers of the psuedopotential did. I used those developed by VASP team and they seem to be well-trusted (except may be for their ultrasoft ones which the VASP team is not developing any more.). To learn more about the kinds of tests they typically do , you may for example download and have a look at some ppt slides on VASP website which they prepared for a workshop in the past. If you have more time you may look up some of their papers as well. Or even better look at the website of the code you are using.