Quantum Monte Carlo made easy

Lead Research Organisation: Imperial College London
Department Name: Dept of Physics

Abstract

Computer simulation is used in many different contexts, but the questions addressed are often surprisingly similar. What happens when many simple objects interact? How does the behaviour of the whole emerge from the simple laws obeyed by its parts? The constituent objects range from electrons and nuclei to girders and cables, but the common aim is to predict the complex large-scale behaviour from the simpler small-scale behaviour. In essence, computers are used to build bridges between length scales.

The use of computers to simulate molecules and solids in terms of their constituent particles is a successful example of this sort of bridge building. The inputs are the identities of the atoms that make up the molecule or solid, and the outputs describe the behaviour of the assembly as a whole. Since the electrons that bond atoms together are quantum mechanical objects and behave more like waves than particles, these simulations have to solve the quantum mechanical Schroedinger equation.

Quantum mechanical simulations are increasingly used across much of the EPSRC's physical sciences portfolio, including all of the following areas: quantum fluids and solids; magnetism and magnetic materials; functional ceramics and inorganics; materials for energy applications; electrochemical sciences; surface science; chemical structure; chemical reaction dynamics and mechanisms; plasmonics; light matter interaction and optical phenomena; photonic materials and metamaterials; cold atoms and molecules; chemical biology and biological chemistry; graphene and carbon nanotechnology; and catalysis. They have also had an impact on parts of the engineering portfolio. The practical importance of quantum mechanical simulation to many of these fields is still quite small, but it is growing fast and its success has been one of the most striking scientific stories of the past twenty years. Few other fields of research have done so much to revitalise and maintain the health of other disciplines.

By far the most common approach to quantum mechanical materials simulation is density functional theory (DFT). This offers a good balance between ease-of-use and precision, but is not quite accurate enough for room-temperature chemistry, biology and biochemistry. This has led to a widespread feeling that it is time to put effort into developing more accurate approaches. One of these is the diffusion Monte Carlo (DMC) method that is the subject of this proposal.

DMC simulations are much slower than DFT simulations but are more accurate and can be used to study large systems of several thousand interacting electrons. Furthermore, unlike DFT codes, a well-written DMC code will run with almost 100% parallel efficiency on even the largest of the world's parallel computers. Large-scale parallelism just happens to suit the method. This proposal seeks support to make CASINO able to run efficiently on the next generation of supercomputers, which will use GPUs (graphics processing units, originally developed for use in graphics cards for computer gamers) for most of the heavy computation. GPU programming presents new challenges and a great deal of work will be required to re-engineer CASINO to use GPUs efficiently. We will also work on making CASINO much easier to use, and further develop our ideas about how to optimize trial wave functions and calculate inter-atomic forces efficiently using DMC. Our aim is to make CASINO into a flexible, efficient, and easy-to-use tool capable of running almost anywhere. CASINO is already the world's most widely used DMC code. We would like the chance to make sure that it retains its lead as the use of DMC takes off over the next decade or two.

Planned Impact

In addition to the academic benefits described in the "Academic Beneficiaries" section, our work will benefit the nation by producing highly-trained people. Computational science "has become generally accepted as the third pillar of science, complementing and extending theory and experimentation" [International Review of Research Using HPC in the UK, EPSRC (2005)] and is widely used in industry as well as academia. Despite the obvious importance of the field, we estimate that the UK produces no more than 100 people per year with a deep and high-level understanding of computational (as opposed to computer) science. The researchers employed by this grant will contribute to the UK's small pool of experts.

More specifically, now that the era of continuously increasing computer clock speeds is coming to an end, improvements in computational power are relying almost entirely on increases in parallelism. Over the next 10 years, every enterprise that requires substantial computer power - a list that includes the electronics, pharmaceutical, transport, banking, financial, defence, internet, and energy industries among others - will be forced to come to terms with massively-parallel massively-multi-threaded computing. For the UK economy to remain internationally competitive, it is important that our research and education systems produce an increasing number of people capable of programming computers with many thousands of cores and using them to solve real-world problems. The researchers employed by this grant and the PhD students trained alongside them will be in a position to render an important service to the nation's economy, whether or not they stay in computational science.

The purpose of the CASINO diffusion quantum Monte Carlo code is to model the properties of molecules and solids on the atomic scale, allowing new materials and molecules to be designed and assessed using a computer without wasting money making and testing the real thing. Materials modelling techniques based on quantum mechanics are only now making their way into industry, but their potential is illustrated by the success of the CASTEP density-functional code, which originated as an academic project at Cambridge University but has been sold commercially since 1995 and accumulated sales of over $20 million.

In the longer run, far beyond the end of this grant, we expect that quantum Monte Carlo methods will also be used in industry, just as density-functional methods are today. This will not happen soon, mainly because quantum Monte Carlo simulations take so much longer to run than density-functional simulations, but density-functional theory is not reliably capable of achieving the accuracy required to study room-temperature chemical and biochemical processes and is unlikely in our view ever to do so. This leaves the way open for more accurate methods such as quantum Monte Carlo.
 
Description We have invented a new way to calculate the forces between the atoms in molecules and solids using the highly accurate diffusion Quantum Monte Carlo (QMC) method. This solves a problem that has been holding back the field for almost 20 years and has allowed us to complete the first ever diffusion-QMC-based molecular dynamics simulation of a chemical reaction. It has never before been possible to study the picosecond by picosecond progress of an interesting chemical reaction with such accuracy.

We have also developed a successful new interactive website for the CASINO project, incorporating access to the code, message boards, blogs, and other social networking goodness.

We have used the CASINO diffusion quantum Monte Carlo code to carry out several high-profile studies of the phase diagram of solid hydrogen and high pressure but low temperature.

Finally, we have developed a new quantum Monte Carlo method, known as density matrix QMC, which allows us to study the full thermodynamics of solids at finite temperature. This has been tested very successfully on the warm dense electron gas and will soon be applied to real warm dense systems.
Exploitation Route The QMC molecular dynamics method is an important breakthrough but it will require a large and long-term effort to implement it in a robust and general way in all of the handful of widely used QMC codes globally. This type of infrastructural work is important for the progress of science but difficult to fund in today's academic environment.
Sectors Aerospace, Defence and Marine,Chemicals,Digital/Communication/Information Technologies (including Software),Electronics,Energy,Manufacturing, including Industrial Biotechology,Pharmaceuticals and Medical Biotechnology

URL http://vallico.net/casinoqmc
 
Description Three of the staff trained under this grant have moved to scientific start-up companies within the past two years. Tom Poole (who was my PhD student) took a scientific software engineering position with Materials Design but has since moved to ROLI. James Spencer (who was our Computational Science Specialist) moved to work as Bioinformatics Scientist for Seven Bridges Genomics Ltd and is now working for DeepMind Ltd. Niccolo Corsini has founded his own company, Sonodot Ltd., which designs and builds an accurate, low-cost, high-performance, and easy-to-use indoor positioning system. All three are using skills developed during this grant to contribute to the UK science and technology industry.
First Year Of Impact 2016
Sector Digital/Communication/Information Technologies (including Software),Electronics,Manufacturing, including Industrial Biotechology,Pharmaceuticals and Medical Biotechnology
Impact Types Economic

 
Description 312,500 node hours per year on Swiss Monte Rosa Supercomputer
Amount SFr. 1 (CHF)
Funding ID 20141010-143952350-568 
Organisation Swiss National Supercomputer Centre (CSCS) 
Sector Academic/University
Country Switzerland
Start 04/2015 
End 03/2018
 
Description ARCHER Resource Allocation Panel - 25,570.9 kAU of high-performance computing time
Amount £1 (GBP)
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 02/2015 
End 02/2016
 
Description Imperial-Kiel QMC collaboration 
Organisation University of Kiel
Department Department of Physics
Country Germany 
Sector Academic/University 
PI Contribution Michael Bonitz and his group at Kiel are our main competitors in applying quantum Monte Carlo to warm dense matter, although they use different methods with different strengths and weaknesses. Bonitz recently reached out to start a collaboration. This is a very new initiative, but he and his group are excellent and I expect it to be productive.
Collaborator Contribution They have organized a small scientific meeting involving three groups to be held in Kiel in mid-April 2016
Impact Several joint publications
Start Year 2016
 
Description Imperial-MIT QMC Collaboration 
Organisation Massachusetts Institute of Technology
Country United States 
Sector Academic/University 
PI Contribution We are working together to apply the Density Matrix Quantum Monte Carlo method recently developed in my group to warm dense matter.
Collaborator Contribution Dr. James Shepherd, who is currently at MIT, alerted us to the importance of warm dense matter and has helped drive the project forward. He has also made significant technical contributions towards improving our methodology. He helped write several successful grant proposals for computer time. He and Professor Troy Von Voorhis have just helped us submit a proposal, which is currently being refereed, for travel and collaborative meeting expenses to a joint MIT-Imperial College funding scheme.
Impact James Shepherd was a co-author on several of the papers reported in this Researchfish submission. He has now moved to take up an Assistant Professorship at the University of Iowa.
Start Year 2014
 
Title HANDE 
Description A code to carry out stochastic quantum chemistry simulations using full configuration-interaction and density matrix quantum Monte Carlo methods among others. 
Type Of Technology Software 
Year Produced 2015 
Open Source License? Yes  
Impact Still working up to 1.0 release (a pre-release candidate is available at https://github.com/hande-qmc/hande) so no impacts yet, but stochastic quantum chemistry is a growing field and we hope the code will be widely used. 
URL http://www.hande.org.uk/
 
Title Local density approximation for warm dense matter: incorporation into libxc. 
Description Libxc is a library of programs for calculating exchange-correlation functionals in density functional theory, and has been incorporated into practically every widely used materials simulation tool based on density functional theory. The most recent version of libxc (version 4.0.4), released in January 2018, includes our new finite-temperature local density approximation, as introduced in Phys. Rev. Lett. 119, 135001 (2017). The libxc implementation is based on the software we provided in C, Fortran and Python at https://github.com/agbonitz/xc_functional. 
Type Of Technology Software 
Year Produced 2018 
Open Source License? Yes  
Impact libxc is used by practically all of the word's well known simulation tools based on density functional theory. Our new finite-temperature local density functional is now available in all of these. 
URL http://octopus-code.org/wiki/Libxc
 
Title Software in Fortran, C and Python to calculate new local density exchange-correlation functional for warm dense matter 
Description Calculates the new finite-temperature exchange-correlation functional for warm dense matter introduced in Physical Review Letters 119, 135001 (2017). 
Type Of Technology Software 
Year Produced 2017 
Open Source License? Yes  
Impact This functional and associated software has now been incorporated into libxc, https://gitlab.com/libxc/libxc/wikis/Functionals-list-4.0.4, which is used by almost every density functional code in the world. 
URL https://github.com/agbonitz/xc_functional
 
Company Name SONODOT LTD 
Description Developing systems to track objects in real time as they move around inside a room or factory. 
Year Established 2017 
Impact Sonodot helps managers increase the efficiency of their material handling operations, gain realtime visibility of their material handling fleet, enforce safety and reduce operational costs
Website https://www.sonodot.com
 
Description Electron behaviour under extreme conditions described for the first time 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact Press release entitled "Electron behaviour under extreme conditions described for the first time" issued by the Imperial College Press Office. This covered the most recent publication to emerge from my collaboration with the University of Kiel and Lawrence Livermore National Laboratory, and was about work that developed from projects started under the "Quantum Monte Carlo made easy" grant. The press release was picked up by at least 10 online media outlets, including materialstoday.com and Cosmos Magazine. Its global reach was demonstrated when I received enquiries from school children in the USA and Australia.
Year(s) Of Engagement Activity 2017
URL https://www.imperial.ac.uk/news/182177/electron-behaviour-under-extreme-conditions-described/
 
Description Kiel physicists can precisely describe the behaviour of electrons under extreme conditions for the first time 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact This press release was issued by our collaborators at the University of Kiel, covering the same paper publicized in a parallel press release by Imperial College. It covered the most recent outcomes of work that developed from a project started under the "Quantum Monte Carlo made easy" grant.
Year(s) Of Engagement Activity 2017
URL http://www.uni-kiel.de/pressemeldungen/index.php?pmid=2017-293-elektronen&lang=en
 
Description New CASINO website 
Form Of Engagement Activity A magazine, newsletter or online publication
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other academic audiences (collaborators, peers etc.)
Results and Impact Built in response to a critical comment about the old CASINO web site made by a referee of our proposal, the new website presents a more professional image of the CASINO program and is helping to build a larger community around it. The website hosts blogs and discussion forums and acts as a one-stop shop for finding out about QMC and downloading CASINO. It also makes clear that, although CASINO is not currently open source, it is available free of charge to any university researcher willing to sign a simple licence agreement.

Within a few weeks of the new site becoming available several hundred people had signed up for accounts; the site now has 544 members in total.
Year(s) Of Engagement Activity 2014
URL http://vallico.net/casinoqmc/
 
Description Press release (warm dense electron gas) 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact Press release reporting the publication and importance of our work on the warm dense electron gas in Physical Review Letters.
Year(s) Of Engagement Activity 2016
URL http://www3.imperial.ac.uk/newsandeventspggrp/imperialcollege/newssummary/news_11-10-2016-17-1-17