Systematic errors due to pseudopotentials
Posted: Fri May 23, 2014 12:18 pm
Having used CASINO for while to evaluate reaction barriers for diatomics on metal surfaces, now is the crunch! Our benchmark for H2 dissociation on Cu(111) agrees well with the best mesurements.
Should it?
We use a Troullier Martins PP that shows a systemtic error around 1kcal/mol in PBE DFT compared to more complete PPs (with 17 instead of 11 active electrons through including the 3p shell). We are worried about this since the error we'd like to claim is 0.5kcal/mol, based on the DMC with twist-averging and extrapolation to cater for finite size effects (these are also large in the DFT calculation).
Now, the PBE DFT barrier is way off the measured one (too low). Could the error due to the PP also be very different in the DMC result and how could it be estimated? A new QMC run with a better PP is likely to take ages and this PP has produced good results for DMC reaction brriers, although the measured values are not within a kcal/mol in the cases we previously studied.
Any help on this would be most welcome.
Should it?
We use a Troullier Martins PP that shows a systemtic error around 1kcal/mol in PBE DFT compared to more complete PPs (with 17 instead of 11 active electrons through including the 3p shell). We are worried about this since the error we'd like to claim is 0.5kcal/mol, based on the DMC with twist-averging and extrapolation to cater for finite size effects (these are also large in the DFT calculation).
Now, the PBE DFT barrier is way off the measured one (too low). Could the error due to the PP also be very different in the DMC result and how could it be estimated? A new QMC run with a better PP is likely to take ages and this PP has produced good results for DMC reaction brriers, although the measured values are not within a kcal/mol in the cases we previously studied.
Any help on this would be most welcome.