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[PGanesh <pganesh@ciw.edu>]

Dear All,

There are two methods to compute the first-derivative wavefunctions 
(ddk), one using iscf = -3 and rfelfd = 2 and the other using berryopt = 
-2 in a non-scf calculation.  The second method uses the 
finite-difference formula introduced by Marzari and Vanderbilt and 
susceptibility has been show to converge much faster with kpt. grid.  So 
will I be correct in saying that second-order derivatives as well (like 
the piezo) will converge much better using this method as opposed to 
method one?  Its probably a moot question, but still wanted to confirm.  
This would probably mean that in the tutorial for getting non-linear 
properties, using "tffield_2.in" would give better piezo. constants than 
"tnlo_2.in + tnlo_3.in".

Thanks a lot.

PGanesh



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