# October 2010

Hi Everyone,

Since Alison is travelling at the moment, I'm moonlighting as the

newsletter coordinator for October. It been a busy month since Stefano

di Gironcoli has been putting together the project results from the

school.

All the best!

Sinead Griffin

**:: CONTENTS ::**

1) Jobs

2) Journal Access from ICTP

3) High Performance Computing School

4) Article of the month: Ab Initio Random Structure Searching

5) Group-project update

**:: JOBS ::**

1) The European Psi-k network keeps a mailing list with an average of 1-2

announcements per day, mostly jobs in electronic structure but also conferences

etc. In order to receive the announcements by email, one needs to register at

the the Psi-k portal http://cselnx9.dl.ac.uk:8080/portal

Registration is free and open to all.

2) The list of opportunities in Condensed Matter Physics at ICTP (including

postdoctoral fellowships and short visits) can be found at

http://cmsp.ictp.it/job-visiting-opportunities

**:: JOURNAL ACCESS FROM ICTP ::**

ICTP maintains, through special agreements with publishers, a free delivery

system for articles to scientists in developing countries. The list of eligible

countries (most, if not all, African countries are included) and information on

how to subscribe can be found at http://library.ictp.it/ejds

Downloads are limited to 3 papers per day, 12 per month and 100 per year.

**:: HIGH PERFORMANCE COMPUTING SCHOOL ::**

CHPC and IBM South Africa are pleased to announce they will be sponsoring a

limited number of students from outside South Africa to attend the High

Performance Computing School and CHPC National Conference 2010 from the 29

November 2010 - 10 December 2010. The course will cover the concepts and

theory of parallel computers, and programming for parallel systems with MPI,

OpenMP and CUDA, using the C, Fortran or python programming languages.

Please see http://www.chpcconf.co.za/index.cfm?x=hpcs for further details.

Deadline for applications: 29 October 2010

**:: RESEARCH ARTICLE OF THE MONTH ::**

Last month's Psi-k Highlight of the Month 'Ab Initio Random Structure

Searching' by Chris J Pickard and R J Needs

http://www.psi-k.org/newsletters/News_100/Highlight_100.pdf

Predicting the configuration that a collection of atoms will adopt is

one of the most common problems for computational scientists. Usually,

as in the case of the silica project, we begin with knowledge of the

structure from direct experimental observation. However in the cases

where experimental evidence is inconclusive or simply does not exist

(which can be the case for extreme circumstances such as very high

pressure or temperature), we don't have a structure to begin with!

Further to this, we could also be interested in metastable structures

that could be obtained by non-equilibrium conditions. A common approach to

this problem is to use simulated annealing where the system is repeatedly

heated and cooled to allow the atoms to find their lowest energy

configuration. Last month's Psi-K highlight of the month explained how

Chris Pickard and Richard Needs developed an alternative approached called

'Ab Initio Random Structure Searching' (AIRSS). It involves creating 'random'

atomic configurations for the material and calculating which arrangements

yield energy minima.

In order to find the lowest energy structures, we need to identify the local

minima of a potential energy surface. The number of local minima has been

shown to increase exponentially with the number of atoms. The task to calculate

the energy of each of the possible local minima essentially becomes impossible.

The idea behind AIRSS is starting with a 'sensible' random structure, where some

initial constraints are opposed on the beginning structure that will hopefully

not bias the calculation to the point where it does not explore enough of the

potential energy surface. General assumptions that are made for all systems are

that they are always fully connected, and so each atom is always connected to at

least one other atom. In addition to this, physically or chemically unreasonable

configurations are discounted, such as having bond lengths that are far too

small. The paper goes on to explain the various constraints that are employed in

studying crystals, cluster and defects.

Two of the big successes of the AIRSS method have involved the search for

metallic hydrogen. Its first big achievement was the prediction of previously

unknown phases of pure hydrogen at high pressures. Previous DFT calculations had

predicted that above about 200GPa, a transition to a metallic phase would occur.

This was contrary to experimental data which showed that it remained insulating

well above 300 GPa. AIRSS calculations showed up several alternative structures

that were in fact insulating. Furthering the search for metallic hydrogen,

calculations on possible high energy phases of aluminium hydride were carried out.

Pickard and Needs predicted a stable semimetallic phase which was later confirmed

by high pressure diffraction experiments. The paper describes the application of

AIRSS to a plethora of other systems, including more elemental and compound

structures at high pressures, the identification of metastable structures, and the

calculation of defects in silicon, showing the extremely wide applicability of the

method.

**:: GROUP PROJECT UPDATE ::**

We now have the data collected from what was calculated in Cape Town!

Richard Martin sent out a full summary of what was done, and so if you

haven't received it please let him know. Below is a brief outline of Stefano

di Gironcoli's collected results from the school.

Calculations of the static equations of state for silica (SiO2) in the Quartz

and Stishovite structure using five different exchange-correlation functionals

were carried out. Two of these were LDA (PZ and VWN) and three were GGA (PBE,

BLYP and WC). Everyone was assigned a functional to calculate and the two

structures were optimized for a pressure range of P=-30 kBar to P=200 kBar. The

results generally agreed with each other with the following results for the

transition pressures:

PZ: 30-35 kBar

VWN: 30-35 kBar

PBE: 95-100 kBAR

WC: 55-60 kBar

BLYP: 155 kBar

These gave the correct ordering of the transition pressures for the various

functionals, however with a difference of ~30 kBar from published results. The

reasons for this offset are now being examined.

The second part of the project involved calculating the dynamical matrices and

phonon frequencies of the quartz and stishovite structures. Enthalpy-Pressure

diagrams were calculated for static, T=0K, T=300K and T=500K. Unfortunately, not

all results for this part were completed due to the more computationally demanding

nature of the calculations.

However, the current results give the transition pressures below:

Temp PZ VWN PBE WC

-------------------------------

Static 26.2 32.0 98.6 56.4

0K 27.0 32.9 98.6 58.2

300K 31.1 36.9 103.2 61.8

500K 37.7 42.2 108.0 66.4

We are now continuing with the project so those that would like to be

involved but haven't contacted Richard should do so as soon as possible.