supported clusters and external pressure

Message

Sonny · #1 Post by **Sonny** » Tue Dec 28, 2010 11:26 am

Dear VASPers;

I am having problems relaxing Rhodium four tetrahedron on Ceria.

What does a positive external pressure and an apparently unstable minimum represent.
The calculation finishes but the structure is unusually unstable; other metals are fine.

------------------------ aborting loop because EDIFF is reached ----------------------------------------

FREE ENERGIE OF THE ION-ELECTRON SYSTEM (eV)
---------------------------------------------------
free energy TOTEN = -1423.585005 eV

energy without entropy= -1423.807009 energy(sigma->0) = -1423.659006

CHARGE: cpu time 50.24: real time 50.25
FORLOC: cpu time 2.17: real time 2.17
FORNL : cpu time 158.03: real time 158.10
STRESS: cpu time 428.43: real time 428.62
FORCOR: cpu time 4.66: real time 4.67
FORHAR: cpu time 3.38: real time 3.38

FORCE on cell =-STRESS in cart. coord. units (eV):
Direction XX YY ZZ XY YZ ZX
--------------------------------------------------------------------------------------
Alpha Z 3000.83565 3000.83565 3000.83565
Ewald 642281.57760642990.06788************ 232.63429 -320.44786 173.59526
Hartree674030.11757674424.80380************ 130.66935 -241.65554 86.02032
E(xc) -5256.45473 -5254.40578 -5265.63471 0.42441 0.18255 0.19365
Local ************************************ -343.02991 588.87695 -256.27557
n-local 390.34453 398.33348 412.55188 2.55269 2.48099 2.08381
augment -102.05578 -113.05767 -96.38722 -3.77686 -2.48162 -1.73639
Kinetic 22836.76058 22798.25324 22812.68923 -10.14320 -7.03218 -3.88506
Fock 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000
-------------------------------------------------------------------------------------
Total 901.38601 944.85527 1207.42773 9.33078 19.92329 -0.00398
in kB 372.62467 390.59446 499.13950 3.85726 8.23611 -0.00164
external pressure = 420.79 kB Pullay stress = 0.00 kB

<span class='smallblacktext'>[ Edited ]</span>

#2 Post by **admin** » Thu Dec 30, 2010 12:08 pm

positive pressure says that the volume as set in POSCAR is to small
BUT
sorry one cannot give an answer based on the data you show. Do you have a Ceria slab with Rh adsorbate? please note that it makes no sense to evaluate the pressure on a unit cell which contains vacuum
are all ionic steps of your calculation fully converged electronically?
what do you mean by 'unusually unstable' ?

Sonny · #3 Post by **Sonny** » Wed Jan 05, 2011 8:08 pm

The lattice constant is optimised: I cant see how the volume is too small.
I have a ceria slab with Rh4 tetrahedron adsorbate.
There is vacuum of 15 angstroms from top Rh.
I find the external pressure is consistently negative and small with other metals.
I say unusually unstable as it converges to a structure physically impossible.
the Rh4 configuration relaxes into a standing Rhombus like structure at about 50 degrees.
I am having difficulty with Rh10 also the top atom is unstable (not fully bound) and mixed Rh , Pd clusters of ten atom with Rh at the vertices; the top Rh sinks this time.
I found a recent experimental to help explain the phenomenon as due to metal donation to f states forming heavy electrons. (the interface is reduced)
http://www.sciencedaily.com/releases/20 ... 131342.htm
At this stage I am not able to examine the bandstructure as I would like too.
I gather Rh nanostructures are quite remarkable in their degeneracy and magetism.
We also have other HRTEM evidence of odd 110 Rh surface of Rh,Pd nanoclusters (~40 atoms) on Cerium dioxide powder, XAFS also lends some insight into amorphous Rhodium phase here.

<span class='smallblacktext'>[ Edited Mon Jan 17 2011, 02:13AM ]</span>