Monthly Archives: November 2013

Report on ‘QMC in the Apuan Alps VIII’ workshop 2013


The eighth “Quantum Monte Carlo in the Apuan Alps” international workshop took place from the 27th of July to the 3rd of August 2013. The participants spent four hours each morning listening to talks on quantum Monte Carlo and related matters, followed by afternoons of mountain walking, caving, canyoning and other activities.

The vitality of research in this field was confirmed by the extremely wide variety of applications discussed, which included high-pressure solid hydrogen, rare gas crystals, biomolecules, ultracold atomic gases, water clusters, water-acene complexes, the design of metal alloys, hexagonal boron nitride, point defects in semiconductors and wide-band gap oxides, molecular crystals such as para-diiodobenzene, proton disorder in ice, nanocrystalline alumina, electron-hole systems, the metallization of solid helium, the binding energy of bilayer graphene, correlation-bound anions and organic diradicals, methane binding in ice clathrates, and (for the first time) the high-Tc  superconducting cuprates. Wagner’s talk on this latter subject – in which he showed highly-accurate DMC calculations of the spin-lattice coupling in the real material (as opposed to a model system) for the first time – was particularly interesting. He found that some lattice degrees of freedom depended strongly on the magnetic state, and that the spin-lattice coupling was removed with 25% doping. Considerable attention was also paid to computing weak interactions such as van der Waals in QMC; talks by Tkatchenko, Benali, Ambrosetti, Deible, Gillan, Jordan, Mostaani were all devoted to this topic in one or another, and showed the very considerable superiority of QMC results over DFT in this area.  As the power of available computers and the sophistication of the algorithms and software increases, it is clear that the size and complexity of the systems that can be treated within QMC continues to rise rapidly. A number of researchers, including Gillan, Wagner, Maezono, Towler, and Alfè are particularly active in pushing the boundaries of what calculations can be done for real systems on large computers.

A number of more technical questions were addressed. These included forces and correlated sampling, finite-size effects, direct incorporation of light nuclei at the quantum level in DMC calculations, maximum probability domains in crystals, and the use of CASINO on systems containing GPUs. A number of speakers such as Tim Mueller stressed the need for using QMC to generate publically-available databases of highly accurate results – for examples of formation energies – and the subsequent requirement for developing methods for doing ‘high-throughput QMC’. QMC is ideal for such databases – which need only be created once and would be useful for a very long time – not only because it produces highly accurate energetics, but because it scales well with processor
number and system size and because it works for ‘everything’ (molecules, metals, insulators, semiconductors..). Mueller’s talk had the interesting and surprising conclusion that – given suitable recipes and workflow improvements -‘by 2016-ish we should be able to calculate QMC energies for every known inorganic material on a single supercomputer in (roughly) about a week‘.

A significant number of people in groups in London and Cambridge are involved with stochastic Monte-Carlo approaches to classic quantum chemistry techniques such as CI and coupled cluster, and these efforts were the subject of a number of interesting talks by Spencer, Thom, and Vigor. The FCI-QMC technique can reproduce the results of full CI calculations (for a given finite basis) essentially exactly but in considerably less time, and it is now possible to calculate energies for systems very much larger than was possible before. The stochastic coupled cluster technique discussed by Thom can reproduce large coupled cluster calculations much more quickly; it is considerably simpler to implement than deterministic coupled-cluster, and is feasible on workstations and very parallelizable.

On a lighter note, notable athletic achievements by the group included the ascents of Monte Matanna, Monte Palodina, Pania della Croce, and Monte Forato, the exploring of the Neanderthal cave (Grotto dell’Onda) and the Cave That Screams, and the penetration of the Turrite di San Rocco and Orrido di Botri canyons. They were the first group to reach the fortress of Trassilico from San Luigi, and ascended a small hill with a nice view near San Pelligrinetto.

Edgar Engel became the first person since pioneers Evans and Towler to reach the Fat Boy Filter in the cave of Cascaltendine by heroically free-climbing the 60 foot wall of the final waterfall chamber. As the opposition fielded their best team (for once) our heroes suffered an embarassing 15-4 hammering by the thrusting young village lads in the fourteenth “Vallico Sotto Against The World” football match (utter humiliation being avoided only by four goals from deadly striker Bartomeu Monserrat-Sanchez). The football was followed by fireworks, dinner, and the Great Rock and Roll Face-off, in which trainee Jerry Lee Lewis impersonator Mike Towler and girly choirboy Alex Thom competed to see who could sing ‘She Was My Baby (He Was My Friend)‘ in the least convincing rock’n’roll style. Needless to say, an almost unanimous vote by everyone except his own loyal daughter Saska voted Towler as by far the worst singer. The other days were concluded with evening discussions and dinner, and on Tuesday, by a trip to the Vallico Sopra Festival where the participants ate local delicacies such as necci, migliecci and pitonca, learned how to dance the Moresca, and observed demonstrations of interesting agricultural techniques.

 

List of participants

Dario Alfè, Alberto Ambrosetti, Sam Azadi, Anouar Benali, Pascal
Bugnion, Mauro Causà, Gareth Conduit, Mike Deible, Andrea Droghetti,
Neil Drummond, Edgar Engel, Elif Ertekin, Matthew Foulkes, Mike Gillan, Richard
Hennig, Kenta Hongo, Ken Jordan, Jonathan Lloyd-Williams, Pierre-Francois Loos,
Matthew Lyle, Martin Krupicka, Ryo Maezono, Natalia Matveeva, Bartomeu
Monserrat-Sanchez, Elaheh Mostaani, Tim Mueller, Richard Needs, Joshua
Schiller, Luke Shulenburger, James Spencer, Jian Sun, Alex Thom, Alexandre
Tkatchenko, Mike Towler, Peter Townsend, Tack Uyeda, William Vigor, Anatole von
Lilienfeld, Lucas Wagner (CANCELLED: Dario Bressanini)


Talks presented

Dario Alfè (d.alfe at ucl.ac.uk)

  • “Graphene on Ir(111): growth and thermodynamics from combined experimental and theoretical methods” – NOT AVAILABLE

Alberto Ambrosetti (ambrosetti at fhi-berlin.mpg.de)

  • “Long range correlation energy from isotropically damped Quantum harmonic oscillators” [PDF]

Sam Azadi (s.azadi at imperial.ac.uk)

  • “Quantum Monte Carlo study of high-pressure solid hydrogen” [PDF]

Anouar Benali (abenali at alcf.anl.gov)

  • “Quantum Monte Carlo calculations of many-body van der Waals forces in rare gas crystals and biomolecules” [PDF]

Pascal Bugnion (pob24 at cam.ac.uk)

  • “Spins, superfluidity, and ultracold atomic gases” [PDF]

Mauro Causà (mauro.causa at unina.it)

  • “Correlated maximum probability domains in crystals” [PPT]

Gareth Conduit (gjc29 at cam.ac.uk)

  • “Concurrent materials design” [PDF] (password protected)

Mike Deible (mjd87 at pitt.edu)

  • “Quantum Monte Carlo studies of water clusters and water-acene complexes” [PDF]

Andrea Droghetti (drogheta at tcd.ie)

  • “A DFT+model Hamiltonian approach to zero-bias transport in nanostructures: work in progress” [PDF]

Neil Drummond (n.drummond at lancaster.ac.uk)

  • “Electronic structure of two-dimensional crystals of hexagonal boron nitride” [PDF]

Elif Ertekin (ertekin at illinois.edu)

  • “QMC for point defects in semiconductors and wide-band gap oxides” [PDF]

Matthew Foulkes (wmc.foulkes at imperial.ac.uk)

  • “Forces and correlated sampling in DMC” [PDF]

Mike Gillan (m.gillan at ucl.ac.uk)

  • “Quantum Monte Carlo benchmarks for weak non-covalent interactions” [PPTX]

Richard Hennig (rhennig at cornell.edu)

  • “Computational discovery and design of materials for energy technologies and electronic devices” – NOT AVAILABLE

Kenta Hongo (kenta_hongo at mac.com)

  • “Finite-size effects in diffusion Monte Carlo simulations of para-diiodobenzene” [PDF] (password required)

Ken Jordan (jordan at imap.pitt.edu)

  • “Quantum Monte Carlo studies of correlation-bound anions and of organic diradicals” [PPT]
    “Proton disorder in ice” – NOT AVAILABLE

Pierre-Francois Loos (loos at rsc.anu.edu.au)

  • “Uniform electron gases: electrons on a ring” [PDF]

Matthew Lyle (mjl78 at cam.ac.uk)

  • “Low density nanocrystalline alumina” [PDF] (password protected)

Ryo Maezono (rmaezono at mac.com)

  • “Studies of electron-hole systems using DMC” [PDF]

Natalia Matveeva (matveeva.na at gmail.com)

  • “Localization of an impurity in a bilayer system of dipoles” [PDF]

Bartomeu Monserrat Sanchez (bm418 at cam.ac.uk)

  • “White dwarf cooling: electron-phonon coupling and the metallization of solid helium” [PDF]

Elaheh Mostaani (emostaani at gmail.com)

  • “Binding energy of bilayer graphene” [PDF]

Tim Mueller (tmueller at jhu.edu)

  • “Quantum Monte Carlo for materials design” [PDF]

Richard Needs (rn11 at cam.ac.uk)

  • “Decomposition and terapascal phases of water ice” [PDF]

Luke Shulenburger (lshulen at sandia.gov)

  • “Status of DMC for condensed phases” [PDF]

James Spencer (j.spencer at imperial.ac.uk)

  • “Full configuration interaction quantum Monte Carlo and coupled cluster Monte Carlo: a framework for stochastic quantum chemistry.” [PDF]

Alex Thom (ajwt3 at cam.ac.uk)

  • “Linked stochastic coupled cluster theory” [PDF]

Alexandre Tkatchenko (tkatchen at fhi-berlin.mpg.de)

  • “Explicit many-body van der Waals corrections to DFT, and how QMC can help to develop them” [PDF]

Tack Uyeda (tueeeda at jaist.ac.jp)

  • “Ohmic contact on diamond semiconductors” [PPTX]

William Vigor (w.vigor11 at imperial.ac.uk)

  • “Accelerating Full Configuration Interaction Quantum Monte Carlo” [PDF]

Anatole von Lilienfeld (anatole at alcf.anl.gov)

  • “Preaching on first principles views on chemical compound space, atom-centered potentials, and statistical learning” [PDF]

Lucas Wagner (lkwagner at illinois.edu)

  • “Can we understand the high-Tc superconducting cuprates from first principles?” [PDF]

Comments

  • “The conference was absolutely incredible. I hope to be back next year.”
  • “Thank you so so much to all of you guys for giving us the opportunity to get
    to know such a wonderful (and almost unreachable, at least at the first
    attempt) place as it is Vallico di Sotto and the whole Apuan Alps area! What
    an experience, it really make us change from the inside. I don’t know if it
    has to be with the lots of fresh air you get there, or the amazing food , or
    the awesome company , or all the wise that concentrated there at the
    Monastery, or the figs …maybe it was all these things together, but at
    least [my husband] and me , we both feel full of energy and optimistic (maybe a
    couple of pounds heavier , doesn’t matter, :o) just ready for what next,
    simply great!”
  • “I didn’t see you yesterday morning before we all left so I just wanted to
    say a huge thank you for putting on ‘QMC in the Apuan Alps VIII’. I had a
    brilliant time attending the lectures and going on the excursions and it was
    a great privilege to be given the opportunity to interact with so many
    distinguished physicists in such a friendly and welcoming environment.”
  • “Thanks again for a great conference, and good luck with the summer school.”
  • “Thanks again for another wonderful conference.”
  • “TTI was very great conference and it was a good experience for me.
    I got many interesting suggestions and friends. I want to attend next TTI !”
  • “We left yesterday early in the morning and didn’t have the chance to say
    goodbye or to thank you and your family once again for your hospitality. I
    really enjoy going to your place every time, and although I must say that I
    have never been to any other conference (and twice to yours), I cannot
    imagine how both science and fun could get any better.”
  • “Thanks for organising another excellent conference. I had a great time.”
  • “Thanks a lot for hosting us in Vallico Sotto!
    You and your family are doing a marvellous thing!
    [My wife] and me really enjoyed the meeting and everything
    else.”
  • “Thanks again for all the efforts, and to Sam and you for including me so
    warmly and openly. This was an amazing event, and I very much look forward
    to any future get-together. Once the school is over, please do enjoy that
    Amarone bottle and tell Sam about the effect of the potential shape on
    electronic level spacing. On my ride back I was wondering if one could
    visualize such excitations, and even make a sound according to the
    frequencies of the levels or their excitations. That might be called the
    music of the electrons. Same thing holds for nuclear wavefunctions of
    course.”
  • “Thanks again for an excellent workshop”
  • “Thank you again for all your wonderful organization and opening your home to
    us.”
  • “We made it home late last night after some long traveling and already miss the valley. Thank you once again for coordinating such an awesome event.”
  • “Thanks for organizing the conference last week — it was an excellent
    scientific event with great activities.”
  • “Thanks very much for organizing the conference, there was a perfect blend of
    strong science and opportunities to discuss ideas with colleagues.”

 

 

 

 

Report on ‘QMC and the CASINO program VIII’ summer school 2013


The eighth international summer school in the “Quantum Monte Carlo and the CASINO program” series took place from Sunday 4th August to Sunday 11th August 2013 and involved 25 students from 16 countries.

The purpose of the school was to provide the students with a thorough working knowledge of the quantum Monte Carlo electronic structure method as currently used in quantum chemistry and condensed matter physics and to show them how to use the Cambridge CASINO QMC program for serious scientific research. The participants spent around four hours each morning listening to lectures on the quantum Monte Carlo method. This was followed by practical examples classes with the CASINO software, and a programme of healthy recreational activities such as mountain walking and cave exploration.

procinto
In the late afternoons various groups of students attacked and conquered Monte Matanna, the cylindrical rock tower of Monte Procinto, the wild Turrite di San Rocco canyon, the legendary Cave of Cascaltendine, the Cave of Fairies of Vallico Sotto, the natural rock arch of Monte Forato, the terrifying ‘Cave That Screams’, the mighty Orrido di Botri canyon, and a small hill near San Pelligrinetto. Special mention should be made of Englishman Matthew Malcomson and Russian Yury Gladush, who spent the week showing off their tremendous athletic abilities and natural courage, by unnecessarily running up very steep mountains just because they could, and by free-climbing various suicidally dangerous slippy rock faces. Following his exploits in the annual “Vallico Sotto against the World” football match, in which he wore only a pair of shorts and no shoes, Malcomson was nicknamed “La Bestia” – the Beast – by locals, and quite right too.

OK so we lost 12-11 on a Golden Goal after a hard fought match against their Junior team, but it’s better than being absolutely caned as is more usual on these occasions. Both men also demonstrated their considerable musical abilities in a Rock’n’roll evening at the Vallico Sotto Star Club, along with Minh Tam Nguyen, Ludmila Szulakowska, Sofia Silva, Mr. Pablo López Ríos (who also managed a 40-minute drum solo during a traditional Vallico Sotto dance night down in the village) and others.

mountains
To check which students have been paying most attention during the lectures and practical classes, we traditionally hold a tough examination on the final day. The highest mark and the award of the prestigious title of “TTI QMC Summer School Champion 2013”, went to Blazej Jaworowski from Wroclaw, for which he was awarded the prize of a 19th century Gentleman’s Telescope. Following our first-ever seaside day trip to Lerici and Portovenere, his triumph was celebrated in the TTI church with fine Barolo-soaked cheese from Lucca and a quality speech from the victor which could have been given by Abraham Lincoln. After Booth shot him.


This year marked the promotion of 17-month-old Jamie Towler to our staff. We are somewhat unhappy to report that he spent the week perambulating about the Institute, clad only in a nappy/diaper, stealing people’s possessions from their rooms, chewing them, and attempting to throw them into the lavatory – before parading into the back of the church and making screeching noises during his father’s lectures. It is to be hoped that – following a rigorous and intensive training program – he might be of considerably more use in future years.

yasmine

Instructors

Mike Towler, Neil Drummond, Pablo López Ríos assisted by Martin Krupicka

Students

Yasmine Al-Hamdani, Tadeusz Andruniów, Can Ataca, Samaneh Ataei, Salvatore Cardamone, Ambesh Dixit, Guillaume Ferlat, Laura Giacopetti, Yury Gladush, Cecilia Goyenola, Blazej Jaworowski, Manuel Perez Jigato, Peter Korir, Matthew Malcomson, Jana Mathauserová, Marcos Menéndez, Elaheh Mostaani, Minh Tam Nguyen, Antonio Noto, Eduardo Menendez Proupin, Samuel Ridgway, Kayahan Saritas, Pedro Silva, Kyrylo Snizhko, Ludmila Szulakowska

people


Lectures presented : slides
(password required)

Mike Towler (mdt26 at cam.ac.uk)

  • “Quantum Monte Carlo : a practical solution to the correlation problem in electronic structure calculations” [PDF]- “The CASINO program : a basic introduction to functionality and input/output” [PDF]- “Three QMC scaling problems: many atoms, many protons, many processors” [PDF]

    “Forces and dynamics. Expectation values other than the energy” [PDF]

Pablo López Ríos (pl275 at cam.ac.uk)

  • “Statistics in quantum Monte Carlo” [PDF]- “Wave functions beyond Slater-Jastrow for QMC” [PDF].  (Animation of nodes [GIF])- “Pseudopotentials for QMC” [PDF]

Neil Drummond (n.drummond at lancaster.ac.uk)

  • “Theory and practice of Diffusion quantum Monte Carlo” [PDF]- “Optimization of many-electron wave functions” [PDF]- “Ewald interactions and finite size effects” [PDF]

    “Quantum Monte Carlo study of the two-dimensional homogeneous electron gas” [PDF]


Practical worksheets and input files


QMC Exam

  • – “Exam” [PDF]
    – “Answers” [PDF]


Student talks

None.

river

 

Comments

  • “I just wanted to express my greatest thanks for this
    wonderful QMC school which in my opinion is the best combination of
    high-level science and great atmosphere ever. Thanks for all the efforts put
    in this!”
  • “I just wanted to say thanks very much again for this absolutely fantastic
    week; the places as well as the people were delightful! I’m sorry I didn’t
    get to say bye to you or Sammy properly (traveling days are always a bit
    hectic) but please tell Sammy and Saska that I really enjoyed their company
    and it was so nice of your family to be so very welcoming.”
  • “After safely arriving home I want to thank you (and all the people who
    helped you) once more for organizing such a great school. I thoroughly
    enjoyed it. First of all, the organizational part was excellent with me not having to
    think about any technical details at all during the school. Adding to this
    the amazingly complete pre-school instructions, I must say that the
    organizational part was really great. The balance between learning activities, sightseeing and Italian dinners (which were as substantial as long 🙂 ) could hardly be better. However, I would hardly survive more than a week of such regime, – and I can
    only imagine how it was to you. Which makes me even more grateful. 🙂
    So in conclusion, I say REALLY HUGE THANK YOU!!!”
  • “I’d like to thank you very much for the great week in Vallico Sotto. It was both instructive and really enjoyable. I had a lot of fun and I couldn’t dream of learning more. I am also glad to have met so many wonderful people!”
  • “Thanks again for organising the conference and summer school! As ever they
    are by far the best workshop & summer school that one could ever hope to
    attend.”
    elaheh

A pseudopotential for ultracold atomic gases

Pascal Bugnion

Pascal Bugnion, Gareth Conduit, Richard Needs

Ultracold atomic gases provide a test bed for the study of fundamental quantum condensed matter phenomena. We are particularly interested in fermionic cold atom gases as these can be used to model collective electronic processes in the solid state.

The dynamics of a dilute ultracold atomic gas is dominated by short-range interactions between the atoms, typically characterized by the scattering length of the inter-particle potential. Modelling this regime is challenging for QMC methods as the wave function diverges when particles coalesce.

Previous attempts at circumventing this divergence have normally involved using an effective potential to model the inter-particle interactions. This effective potential is constructed to reproduce some of the features of the true potential while avoiding its pathological behaviour. We investigate the applicability of the wide range of methods developed by the electronic structure and chemistry community to the development of an effective potential for interactions between cold atoms.

Pairing wave functions for quantum Monte Carlo

Pascal Bugnion

Pascal Bugnion, Pablo López Ríos, Richard Needs

Diffusion Monte-Carlo (DMC) has been used to describe many real and model systems, usually displaying impressive agreement with experiments while maintaining favourable scaling with system size.

The accuracy of diffusion Monte-Carlo is ultimately limited by the so-called ‘fixed-node’ error, which depends on the quality of the trial wave function used as input to the algorithm.

Typically, the DMC trial wave function is generated in a variational quantum Monte Carlo (VMC) calculation, where it is optimized to minimize the VMC energy. It is heuristically believed that trial wave functions with lower VMC energies will have better nodal surfaces.

To reduce fixed-node error, we therefore need VMC trial wave functions whose functional form is close to the ground state wave function. In this project, we will study a class of such wave functions: pairing wave functions. Pairing wave functions contain parameters which can be optimized variationally, allowing in principle the determination of a better nodal surface. This produces lower DMC energies and more accurate results. We shall investigate how well pairing functions behave in larger systems, looking in particular at size extensivity.