EOS and high cutoff for systems with H atoms

Message

lawson · #1 Post by **lawson** » Sun Feb 19, 2006 2:04 am

I am calculating the equation of state (E vs V) for KCl and
KOH and I had two questions:

1. I understand that GGA overestimates lattice constants and
therefore LDA is better for these calculations. Is the same true for the entire EOS?

2. To converge the energy (mH) for KOH it was necessary to have a very high cutoff ~1200 eV. Is this reasonable? Does the presence of H present any special problems? For KCl, I got convergence at 400 eV.

Thanks for any input!
John

#2 Post by **admin** » Mon Feb 20, 2006 1:39 pm

1) I would recommend to use PAW-PBE potentials, because LDA will probably overbind and give too small lattice constants (as a preliminary test, please check which potentials give the best ground state lattice constant compared to experiment)
2) yes, the short O--H distance will require an extremely high cutoff to reach convergence. Please use hard O-pseudopotentials for that calculation)