The accuracy depends on the property and the computational method used:
DFT band gaps calculated with GGA (PBE) functionals systematically underestimate experimental values by 30-50%. For example, silicon's computed gap is typically 0.6 eV vs. the experimental 1.12 eV. JARVIS provides MBJ-corrected band gaps that are more accurate for many materials.
GGA formation energies are generally accurate to within 0.1-0.2 eV/atom for most inorganic compounds. The Materials Project applies energy corrections for certain elements (O, S, N) and oxidation states.
This is a relative quantity and benefits from error cancellation. Ehull values < 0.025 eV/atom reliably indicate thermodynamically stable phases. The error in Ehull is typically 0.02-0.05 eV/atom.
Computed lattice parameters are typically within 1-3% of experimental values for GGA calculations. Interatomic distances are reliable for most bonding environments.
Use computed properties for screening and trend identification rather than precise quantitative predictions. Always validate top candidates with higher-level calculations (HSE06, GW) or experimental measurements.