Nature Materials訪談：倫敦 Thomas Young 中心材料理論模擬

材料牛注：材料建模對于材料科學的理論和實驗發展非常重要，基于此有許多科學家和機構研究材料的計算與理論模擬，如：倫敦的Thomas Young 中心。他們試圖通過計算和模擬材料中的電子運動來對材料的性能進行預測、調控，為先進新材料的制備以及其相關學科的發展提供理論支撐。

采訪者（Q）：Nature Materials

訪談對象（A）：Angelos Michaelides ，倫敦大學學院理論化學教授，Thomas Young 中心聯合主任

圖為Angelos Michaelides

Q：你為何對材料建模感興趣？

How did you become interested in modelling?

A：在?Queen’s University Belfast 讀本科的時候就被建模所吸引，在原子尺度上得到清楚的信息，可以以一個非常明確的方式來測試事物，我喜歡這樣的想法。理論和模擬吸引著我用最簡單的方法來得到信息。

I was attracted by modelling during my?undergraduate days at Queen’s University?Belfast. I liked the idea of getting very clean?information at the atomic scale and being able?to test things in a?very well-defined manner.Theory and simulation appeared to me to be?the easiest way to access this information.

Q：你現在致力于研究什么？

What are you currently working on?

A：我和我的團隊目前致力于研究材料建模方面的一系列項目，這包含很多問題。如對于DNA里面氫鍵的理解，表面分子參考資料數據的確定，高壓材料。目前，我們有個歐洲研究委員會資助的項目，這個項目的主要目的是研究冰在分子尺度的形成及其雜質在這個過程中的作用。

超凈水很難結冰，在0攝氏度以下很容易形成過冷狀態。0攝氏度是熱力學上相轉變溫度，通常情況下水在0攝氏度下會結冰，這是因為雜質在水的結冰過程中起到了催化劑的作用，降低了形核的能壘。目前，沒有理論可以預測材料在成核過程中是好還是壞。我們利用模擬的辦法來解決這一問題，篩選不同種類的材料，在基底上進行模型研究。發展這一項基礎研究，在理論上可以使我們控制冰的形成，更好的理解云的形成。

My team and I are working on a range of projects in materials modelling, covering different questions, from the understanding of hydrogen bonding in DNA, to the determination of accurate reference data for molecules on surfaces, to high-pressure materials. At the moment, we also have a project funded by the European Research Council aimed at understanding ice formation at the molecular scale, and the role that impurities play in this process.

Ultraclean water is surprisingly difficult to freeze and can easily be supercooled to well below 0 °C. The reason that water typically freezes at 0 °C, the thermodynamic phasetransition temperature, is the presence of impurities acting as catalysts, which reduce the free-energy barrier of nucleation. At the moment, there is no theory able to predict which materials are good or bad at nucleating ice.

We are working on using simulation techniques to address this by,for example, screening different types of materials as well as doing model studieson generic substrates. Developing this fundamental insight could in principle also allow us to control ice formation, and better understand cloud formation.

Q：你和實驗小組合作嗎？

Do you collaborate with experimental groups?

A：當然，我們很大一部分的論文是和實驗者合作完成的。這種合作在理解問題上互利互惠，例如，界面上的水的問題。水分子之間相互作用和環境之間的作業都很弱，通過調節氫鍵和范德華力之間的平衡，可以形成非常復雜的相圖，相圖上大約有17種相。這種豐富的行為意味著即使是單原子組成的一個平面我們很難提前知道水會形成什么樣的結構。因此，實驗者的對我們的指導非常有利，他們也經常揭示一些有趣或者令人驚訝的結構。我們的觀點和理論對實驗者來說也很有用，例如，通常最好的掃描探針測試手段都無法探測表面吸附層完全溶解的結構。

Yes, a large fraction of our papers are joint works with experimentalists. These collaborations are of great mutual benefit in understanding, for example, water at interfaces. Water molecules interact weakly with each other and their environment through a balance of hydrogen bonding andvan der Waals dispersion forces resulting in an extraordinary complex phase diagram, for example, with about 17 different ice phases. This rich behaviour means thateven on an atomically flat surface it is difficult to know in advance what kind of structures water will form.

Guidance from experimentalists is therefore of huge benefit to us, and it also often reveals interesting or surprising structures. We’re also of use to the experimentalists, for example, even the very best scanning probe measurements struggle in general to fully resolve structures of adsorbed layers on surfaces。

Q：什么是Thomas Young 中心，你在里面起著什么樣的作用？

What is the TYC, and what is your role?in it?

A：Thomas Young 中心是一個以倫敦為基礎的研究中心，主要研究材料的理論和模擬。這個中心是將我工作的倫敦大學學院、倫敦大學瑪麗皇后學院、倫敦國王學院、帝國理工學院聯合起來的一個組織，實際上它是一個民間組織，今年是它組建十周年，它致力于讓倫敦成為世界上材料理論計算和模擬最好的地方。利用在倫敦的一大批杰出的研究者，希望通過合作而不是競爭來達到這個目標。

The TYC is a London-based centre for the?theory and simulation of materials. It brings?together researchers from UCL, where I?am based, King’s College London, Imperial?College and Queen Mary University of?London. It is essentially a grass-roots?organization, now in its tenth year, that aims?to make London one of the best places in?the world to do theory and?simulation of?materials. We hope that by collaborating,?rather than competing, we can achieve this,?and in so doing, exploit the critical mass of?outstanding researchers in London.

這個研究中心有著公平的管理團隊，團隊是由分別來自各個大學的聯合主任組成的。Alessandro De Vita?是倫敦國王學院的聯合主任，Arash Mostofi?是帝國理工學院方面的負責人，Martin Dove?是倫敦大學瑪麗皇后學院方面的負責人，Alex Shluger和我是倫敦大學學院聯合主任。Thomas Young 中心所有的管理結構有來自每一個成員大學的人，這點很贊！我們決定的策略和目標對于每一個成員大學都是公平的，杜絕不同大學之間不健康的競爭。很幸運的是，我們得到了很多極好的行政支援。

The TYC has a fairly flat management?team formed of co-directors from the?member universities. Alessandro De Vita?is co-director at King’s College London,?Arash Mostofi leads the work at Imperial?College, and Martin Dove at Queen Mary.?Alex Shluger and myself are co-directors?at UCL. One of the nice things about the?TYC is that all colleges are represented in?this management structure. We all decide?the strategies and goals for the centre on?an equal basis and there is no (unhealthy)?rivalry between the different colleges. We?are also fortunate to have some excellent?administrative support

Q：Thomas Young 中心實際上是怎么運行的？

And how does the TYC work in practice?

A：在日復一日的工作基礎上，Thomas Young 中心致力于進行世界上一流的科研活動，以此來使我們的每一個成員大學，特別是我們的學生受益。我們注入了很多的精力與財力進去，給學生提供材料理論與模擬方面優質的教育和盡可能好的學術環境。他們在所有的大學里享受濃郁的學術氣息，我們開設了Thomas Young?碩士課堂，在這個課堂里面講課的老師都是世界上著名的研究者，這些老師會在這個課堂里面講授他們所擅長的領域知識。在這片學術的沃土之上，很容易見到年輕的學者，這片沃土也發展了很多新的研究方法和研究計劃。確實，在倫敦，Thomas Young 中心在培養不同領域的相互合作方面很有用處。

On a day-to-day basis, the TYC aims to run?top-class events and activities for the benefit?of all our members and in particular our?students. We put a lot of value into giving our?students an outstanding education in theory?and simulation of materials and in providing?the best possible intellectual environment for?them. They get exposed to academics across?all the colleges, and amongst other things we?run the so-called TYC Master Classes, where?world-leading academics fly in to London?to teach specialist classes on their particular?area of expertise. In this fertile environment?young academics can meet people easily and?develop new research ideas and projects.?Indeed the TYC has been very useful in?fostering collaborations between different?groups across London.

Thomas Young 中心另外的一個核心作用就是提供了連貫性的方式，與工業生產緊密結合起來。許多有趣的問題需要從建模來入手解決，但是一些工業家不知道哪個學者能夠解決這樣的問題。Thomas Young 中心有大約100個研究團隊組成，在材料的現在理論和模擬方面非常擅長，所以，在實質上我們可以實現一站式的服務，促進創造工業家與我們學者之間的合作伙伴關系。與此相類似的，我們可以同大的跨國公司和組織進行高度戰略性的相互交流與合作，我們Thomas Young 中心在這方面做的很成功，例如同BP公司，Rolls-Royce公司和美國的很多國家實驗室都有很好的合作伙伴關系。

Another core aspect of the TYC is that we provide a coherent way to engage with industries. There are many interesting problems that require input from modelling, but industrialists do not always know who the right academic to approach is. The TYC is formed of about 100 research groups with expertise in almost all areas of modern theory and simulation of materials. So, in essence we can act as a ‘one-stop shop’and facilitate the creation of partnerships between industry and our academics. Similarly, because we are a large entity, we can engage strategically at a high level with large international companies and organizations. The TYC has successfully?done this, for example with BP, Rolls-Royce and several national labs in the US.

Q：你認為Thomas Young 中心會成功嗎？

Do you consider the TYC to be?successful?

A：當然會！我們已經做成了許多一開始沒有預料到的事情，這十年的發展是表明我們做的事情是正確的，我們是有用的也是有需要的。但是，當然我們可以做得更好。特別地，我非常想看到Thomas Young 中心能夠工業的項目中進行的更加深入，在英國能夠作為一個杰出的區域中心被外界所認可。我們這個團體里面的研究者是非常有熱情、有干勁的，為了中心的成功，我們希望大學能夠提供更誘人的環境來吸引最杰出的青年研究學者來到倫敦。

中心的研究者經費很大程度還是來自國家研究委員會，歐洲計劃和其他的基金組織。這些組織都認為材料建模是現代科學研究必不可少的一個部分，倫敦是世界上研究材料建模最好的地方之一。我們還很大程度上依賴高性能的計算，而大學和研究委員會需要在本地區的水平上繼續投資。如果在英國，特別是倫敦，仍然想在理論和模型方面保持著競爭力，那么對于高性能計算的投資是很重要的。如果沒有合適的投資的話，無疑國家流失最好的建模研究者，而這會使國家進行更大的投資。

Yes, and we have managed to do much more than anticipated at the outset. Simply having lasted for ten years is a sign that we are doingsomething right, that we are useful and that we are needed. But, of course, we can always do better. In particular I would like to see the TYC involved in more industrial projects and to get more recognition as a regional centre of excellence in the UK. Our community of researchers is enthusiastic, active and engaged, and for the success of the TYC we need to ensure that our universities provide an environment that attracts the best and the brightest young researchers to London.

Our members also rely heavily on support from the national research councils, European programmes and other funding agencies; such organizations are increasingly recognising that materials modelling is an indispensable part of modern scientific research and that London is one of the very best places internationally for this sort of research. We also rely very heavily on high-performance computing (HPC), and this requires continued investment from our universities at a local and regional level, and from the research councils. Investment in HPC is critical if the UK in general, and London in particular, are to remain competitive in theory and simulation; without appropriate investment there is no doubt that the country will lose more of our best modelling researchers to countries making greater investments.

Q：回到材料建模，它當前的局限性和挑戰是什么？

Going back to materials modelling, what are its limitations and current challenges?

A：采用更加精確和更加計算有效的方法，我們通常的目的在于模擬更大更加真實的體系，探索長時間跨度的過程事件。20年前，你也許已經得到了一個類似的答案，但是由于理論和模型軟件的發展，我們的能力得到顯著的提高。顯然，計算能力的提高對我們收益匪淺。試想一下我們運用密度泛函理論來進行電子結構的計算，我們總是在進步，現在我們使用所謂的線性縮放密度泛函理論使得計算成百上千甚至是上百萬的原子的電子結構成為了可能。

通常情況下，線性縮放密度泛函理論的應用還是很少的，而密度泛函理論只能計算成百、成千的原子電子結構。例如，我們可以模擬，在一個平面很小的一部分上大約有100個水分子，或者在一個有著很明確的缺陷的表面。這是在真實物理系統里面很明顯的一個簡化，但是它抓住了足夠的物理特征，給人們提供了很有用的信息。在這樣的體系里面，可以進行一個短的從頭算來進行分子動力學模擬，或許可以達到100 ps。偶然情況下，這足夠用來獲得關于這個體系特征的有用信息，但是，如果一個人對于模擬化學過程和晶體在水里的溶解很感興趣的話，例如晶體在水里的溶解，則提高采樣的技術是有必要的。當然，比系統尺寸大幾個數量級的經典分子的動力學過程是可以看到且這個過程最長可達到毫秒級別。

將經典和量子方法結合起來的，就是所謂的QM/MM 技術。在未來，一個有趣的方面是它的應用和經典分子潛力的發展，這種潛能可以通過機器學習從頭算數據來訓練。在犧牲經典分子潛能的代價下，這些機器學習潛力提供量子準確性。

We generally aim to model larger and more realistic systems and to explore processes that happen on longer timescales, with more accurate and more computationally efficient approaches. Twenty years ago, you would have gotten a similar answer but our capability has increased dramatically, primarily due to the development of improved theories and simulation software. Obviously, improvements in computational capacity have also benefited us greatly. Advances are always being made, but let’s assume that we are talking about an electronic structure calculation with density functional theory (DFT). It is now possible to use so-called linear scaling DFT methods to do calculations of hundreds of thousands or even millions of atoms.

In general though, linear scaling DFT is still quite rare and DFT calculations on a routine basis typically involve a few hundreds or thousands of atoms. For instance, we can model a few hundred water molecules on a small segment of a flat surface or a surface with some very well-defined defects. This is obviously a major simplification of the real physical system, but the hope is that it captures enough of the physics to provide useful insight. With such a system one can run short ab initio moleculardynamics simulations for perhaps up to 100 ps. Occasionally, this can be sufficient to obtain useful information about the system of interest, but if one is interested in modelling chemical processes, for instance how a crystal might dissolve into water, thenenhanced sampling techniques are needed. With classical molecular dynamics it is, of course, possible to look at system sizes several orders of magnitude larger and to run dynamics up to milliseconds.

There are also hybrid techniques that combine classical and quantum approaches, socalled QM/MM techniques. An interesting area for the future is the application and development of classical potentials that have been trained through, for example, machine learning on ab initio data. These machine learning potentials offer the promise of quantum accuracy at the cost of a classical potential.

Q：這些局限性如何影響你們自己的研究？

How do these limitations affect your own research?

A：為了研究材料的電子性能，密度泛函理論是目前研究的重點。原則上它包含近似，這就意味著密度泛函理論無法解釋范德華色散力。事實證明這些力對于水分子之間、水分子與界面的鍵合非常重要，許多研究小組對此做出了令人激動的發展，這些發展在某種程度上克服了范德華力無法應用密度泛函理論的問題。然而，在對待不同相的水使用密度泛函理論計算分析時，精確性依然不夠好。

在經典分子模擬動力學基礎上，我們處于僅僅能預測多相的冰形成速率的階段。然而，在這個模擬過程中精細的電子結構和極化效應都沒有考慮進來。我們在從頭算的精度水平計算成核速率（利用精確的從頭算理論），對待任何一個絕對成核速率的計算都要萬分小心。目前，如果我們想解釋一個特定的礦物在成核后性能好還是壞，就需要一個完整的博士學位。越準確的電子結構模擬，精細的機器學習潛力，再加上更加有效的采樣技術，一個博士生也許在幾年的時間內就能篩選出成百上千種材料，這將極大的增加我們探索有用而且有預測性的理論的機會。

The workhorse technique that we have in order to look at the electronic properties of materials is DFT. Although exact in principle, in practice it contains approximations which mean that traditionally DFT does not account for van der Waals dispersion forces. It turns out that these forces are particularly important to the binding between water molecules and between water molecules and surfaces. Various groups have made exciting developments that have allowed for the problem of van der Waals dispersion forces within DFT to be overcome to some extent. However, the accuracy of DFT for treating water in all its various phases and at surfaces is still not as good as we would like.

We are now just about at the stage where we can make predictions about heterogeneous ice formation rates on the basis of simulations with classical molecular dynamics. However, subtle electronic structure and polarization effects are not taken into account in such simulations. Untilwe can compute rates at an ab initio level (and with an accurate ab initio theory at that), we must treat any predictions of absolute nucleation rates with the utmost caution.

At the moment, if we want to explain why a specific mineral is good or bad at nucleating ice, it basically takes an entire PhD. With more accurate models of the electronic structure coupled with sophisticated machine learning potentials and more efficientenhanced sampling techniques, maybe the PhD student that comes along in a few years will be able to screen thousands of materials — this would certainly greatly increase our chances of developing a theory that is really predictive and useful.

Q：正如你前面所說的，Thomas Young 中心與工業有著緊密的聯系，你感覺到材料建模對工業生產的影響了嗎？

As you have explained before, the TYC?actively engages with industry. Do you feel?that materials modelling has had industrial?impact?

A：新的證據表明，材料建模為工業的發展提供了助推劑。在許多領域都有這樣的例子，多相催化就是其中一個例子。在這個領域里，材料建模（主要是密度泛函理論）被用于發展基礎研究，這樣的研究能夠用來預測不同種類的反應催化劑、發開新合金來提高催化性能。還有很多工作與材料的發現相關，人們希望辨別和篩選出具有理想功能的材料。例如，這樣的方法使得電池行業取得了許多進展，這些進展都已運用到了具體的生產實踐當中了。Stefano Curtarolo 和他的同事發表了一篇很有趣的綜述（Nature Mater.12, 191-201; 2013），這篇綜述講的是如何利用高通量的計算來影響不同領域的科學發展，還有人已經寫了一個高通量的計算如何影響工業生產的文章。

There is now clear evidence that materials modelling is providing real insight leading to improvements in industry. There are examples in many fields, heterogeneous catalysis is one of them. In that field, materials modelling (mainly DFT) was used to develop the fundamental insight that allows quantitative predictions to be made about the reactivity of different catalysts, and this insight has since been used to develop new alloys that have improved catalytic performance. There is also a lot of work associated with materials discovery in which people hope to identify and screen materials with desired functionalities.

This approach has led, for example, to some improvements in battery technology that are being exploited in industry now. Stefano Curtarolo and colleagues, for example, wrote an interesting Review Article (Nature Mater. 12, 191–201; 2013) on how high-throughput computation has had an impact in different areas of science, and others have also written about this within an industrial context.

Q：在你的腦海里，有沒有想過你研究的領域會影響未來的發展？

Do you have in mind any industry or field where you think your research may have an impact in the future?

A：BP公司已經資助我們在腐蝕和成核方面的研究工作，我認為他們已經深刻的意識到模擬的重要性，這將給他們帶來巨大的價值。我目前的是更好的理解和控制冰的形成，這個將在許多領域有著潛在應用，如航空、食品加工、低溫貯藏。我希望這項工作能夠最終應用到上述的每一個領域里。在氣候和大氣科學領域，我希望我的建模可以有更大的影響，這就是氣候的模擬。當前，大氣科學家在宏觀領域的研究收集的數據和表面科學家對于表面水原子結構的理解之間還存在著差距。我認為建模可以充當一個橋梁的作用，總體來說我認為這個領域的建模還有待開發。理解大氣冰的成核是很重要的，因為這會影響全球云量和類型，本質上來說就是影響冰與水的相對含量，與這相關的是熱量的反射與保存，這會影響地球的溫度。

BP has funded some of our work on corrosion and nucleation, and I think already they see value in the fundamental understanding that our simulations have provided. My current research into a better understanding and subsequent control of ice formation could see application in several sectors, ranging from the airline to the food industries to cryopreservation. I hope that this work will eventually lead to useful insight that can be applied in each of these areas. Another very relevant area where I hope our modelling will have more impact is in climate and atmospheric sciences, and subsequently in climate modelling.

At the moment there is a serious gap between the extremely valuable but largely macroscopic data collected in field studies by atmospheric scientists and the atomistic insight obtained by surface scientists on the structure of water at interfaces. I think modelling can act as a bridge, and in general I think modelling is still underexploited in this area. Understanding atmospheric ice nucleation is important because it impacts on the amount of cloud cover across the globe and on the type of clouds; basically the fraction of water relative to ice. This is relevant to the amount of heat that is reflected out or kept in, and therefore to the temperature of the planet.

原文參考地址：Materials modelling in London

素材：朱德杰 ?編譯：朱德杰

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