[CPMD-list] geometry optimization - isotopes and convergence doubt

[Mark Kosmowski]

Dear CPMD Community:

I am taking the advice given earlier by Axel and running some
convergence vs. cutoff testing and have a couple questions.  I will
briefly describe my calculations and then ask my questions.

I have a system with two chemically unique hydrogens.  We have
vibrational data for all hydrogens as 1H and for one of the hydrogen
species being deuterium, so I am running calculations specifying
isotopes.  When I first set up a batch script, it was easiest for me
to do both geometry optimization and vibrational analysis for each 1H
and 2H forms.  Prior to deciding to do a cutoff energy convergence
test, I had run a k point study at 150 Ry (chosen from a non-optimized
single point energy vs. cutoff plot).

At this cutoff, I found that I needed to use (minimally) 2 k points in
each dimension to achieve zero negative frequencies, so I am
performing the convergence vs. cutoff energy using this k point
condition.  I am now running additional tests at 50, 100, 200 and 250
Ry to examine geometry and vibrational convergence.  Is this a proper
strategy?

At the 150 Ry cutoff, I notice a 0.00017 A bond length difference (out
of 1.57 A bond length) in the bond of most interest between the 1H and
2H geometries.  There is also a 0.000244 (out of -32.65) total energy
difference.  Is this expected behavior?  If so, do these differences
have physical meaning?

Lastly, at the 50 Ry cutoff for the 2H geometry optimization, I am
seeing several instances of the following message prior to the final
geometry:

 !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
 !! UPDRHO| B2MAX IS VERY HIGH                                 !!
 !! UPDRHO| DOUBT ON CONVERGENCE. WE TRY ONCE AGAIN            !!
 !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
           1     2.380E-04  -32.291104   0.000E+00   1.00    1.78
           2     2.216E-04  -32.291104   6.933E-08   6.00    6.22

I am assuming that this just means that 50 Ry is not a high enough
cutoff energy for my system regardless what the parameter vs. cutoff
plots look like.  Is this a correct conclusion?

Thank you,

Mark Kosmowski

Graduate Student
Physical Chemistry
Syracuse University
Syracuse, NY, United States