Dr. David Lopez, Universidad de Córdoba, Spain
The need to find easily renewable and environmentally friendly energy sources alternative to the traditional fossil fuels is nowadays a global quest. The solar energy is a promising candidate and organic solar cells (OSCs) have attracted attention. In this collaboration with Merck, E-CAM scientists have used electronic structure calculations to study how a key magnitude – the HOMO-LUMO band gap – changes with respect to the molecular disposition of the donor-acceptor molecule pair.Continue reading…
CLstunfti is an extendable Python toolbox to compute scattering of electrons with a given kinetic energy in liquids and amorphous solids. It uses a continuum trajectory model with differential ionization and scattering cross sections as input to simulate the motion of the electrons through the medium.
Originally, CLstunfti was developed to simulate two experiments: A measurement of the effective attenuation length (EAL) of photoelectrons in liquid water  and a measurement of the photoelectron angular distribution (PAD) of photoelectrons in liquid water . These simulations were performed to determine the elastic mean free path (EMFP) and the inelastic mean free path (IMFP) of liquid water .
The EMFP and IMFP are two central theoretical parameters of every simulation of electron scattering in liquids, but they are not directly accessible experimentally. As CLstunfti can be used to determine the EMFP and IMFP from experimental data, and as it can be easily extended to simulate other problems of particle scattering in liquids, it was decided to make the source code publicly available. For this purpose, within the E-CAM module, the necessary steps were taken to make CLstunfti a useful toolbox for other researchers by providing a documentation, examples, and also extensive inline documentation of the source code.
CLstunfti is available at https://gitlab.com/axelschild/CLstunfti .
References Suzuki, Nishizawa, Kurahashi, Suzuki, Effective attenuation length of an electron in liquid water between 10 and 600 eV, Phys. Rev. E 90, 010302 (2014)  Thürmer, Seidel, Faubel, Eberhardt, Hemminger, Bradforth, Winter, Photoelectron Angular Distributions from Liquid Water: Effects of Electron Scattering, Phys. Rev. Lett. 111, 173005 (2013)  Schild, Peper, Perry, Rattenbacher, Wörner, Alternative approach for the determination of mean free paths of electron scattering in liquid water based on experimental data, J. Phys. Chem. Lett., 11, 1128−1134 (2020)
Traditionally high-throughput computing (HTC) workloads are looked down upon in the HPC space, however the scientiﬁc use case for extreme-scale resources required by coordinated HTC workﬂows exists. For such cases where there may be thousands of tasks each requiring peta-scale computing, E-CAM has extended the data-analytics framework Dask with a capable and eﬃcient library to handle such workloads.
The initial motivation for E-CAM’s High Throughput Library, jobqueue_features library , is driven by the ensemble-type calculations that are required in many scientiﬁc ﬁelds, and in particular in the materials science domain. A concrete example is the study of molecular dynamics with atomistic detail, where timesteps must be used on the order of a femto-second. Many problems in biological chemistry and materials science involve events that only spontaneously occur after a millisecond or longer (for example, biomolecular conformational changes). That means that around 1012 time steps would be needed to see a single millisecond-scale event. This is the problem of “rare events” in theoretical and computational chemistry.
Modern supercomputers are beginning to make it possible to obtain trajectories long enough to observe some of these processes, but to fully characterize a transition with proper statistics, many examples are needed. In such cases the same peta-scale application must be run many thousands of times with varying inputs. For this use case, we were conceptually attracted to the Dask philosophy : Dask is a speciﬁcation that encodes task schedules with minimal incidental complexity using terms common to all Python projects, namely dicts, tuples, and callables.
However, Dask or it’s extensions do not currently support task-level parallelization (in particular multi-node tasks). We have been able to leverage the Dask extension dask_jobqueue  and build upon it’s functionality to include support for MPI-enabled task workloads on HPC systems. The resulting approach, described in the rest of this piece, allows for multi-level parallelization (at the task level via MPI, and at the framework level via Dask) while leveraging all of the pre-existing eﬀort within the Dask framework such as scheduling, resilience, data management and resource scaling.
E-CAM’s HTC library was created in collaboration with a PRACE team in Wrocław, and is the subject of an associated white paper . This effort is under continuous improvement and development. A series of dedicated webinars will happen in the fall of 2020, which will be an opportunity for people to learn how to use Dask and dask_jobqueue (to submit Dask workloads on a resource scheduler like SLURM), and to implement our library jobqueue_features in their codes. Announcement and more information will soon be available at https://www.e-cam2020.eu/calendar/.
The jobqueue features library  is an extension of dask_jobqueue  which in turn utilizes the Dask  data analytics framework. dask_jobqueue is targeted at deploying Dask on several job queuing systems, such as SLURM or PBS with the use of a Python programming interface. The main enhancements of basic dask_jobqueue functionality is heavily extending the conﬁguration implementation to handle MPI runtimes and diﬀerent resource speciﬁcations. This allows the end-user to conveniently create parallelized tasks without extensive knowledge of the implementation details (e.g., the resource manager or MPI runtime). The library is primarily accessed through a set of Python decorators: on_cluster, task and mpi_task. The on_cluster decorator gets or creates clusters, which in turn submit worker resource allocation requests to the scheduler to execute tasks. The mpi_task decorator derives from task and enhances it with MPI speciﬁc settings (e.g. the MPI runtime and related settings).
In Fig. 1 we show a minimal, but complete, example which uses the mpi_task and on_cluster decorators for a LAMMPS execution. The conﬁguration, communication and serialization is isolated and hidden from user code.
Any call to my_lammps_job results in the lammps_task function being executed remotely by a lammps_cluster worker allocated by the resource manager with 2 nodes and 12 MPI tasks per node. The code can be executed interactively in a Jupyter notebook. To overlap calculations one would need to return the t1 future rather than the actual result.
The library can eﬀectively handle simultaneous workloads on GPU, KNL and CPU partitions of the JURECA supercomputer . The caveat with respect to the hardware environment is that you need to be able to have a network that supports TCP (usually via IPoIB) or UCX connections between the scheduler and the workers (which process and execute the tasks that are queued).
With respect to the software stack, this is an issue highlighted by the KNL booster of JURECA. On the booster, there is a diﬀerent micro-architecture and it is required to completely change your software stack to support this. The design of the software stack implementation on JURECA simpliﬁes this but ensuring your tasks are run in the correct software environment is one of the more diﬃcult things to get right in the library. As a result, the conﬁguration of the clusters (which deﬁne the template required to submit workers to the appropriate queue of the resource manager) can be quite non-trivial. However, they can be located within a single ﬁle which will need to be tuned for the available resources. With respect to the tasks themselves, no tuning is necessarily required.
We see ∼90% throughput eﬃciency for trivial tasks, if the tasks executed for any reasonable length of time this throughout eﬃciency would be much higher.
The library is ﬂexible, scalable, eﬃcient and adaptive. It is capable of simultaneously utilising CPUs, KNL and GPUs (or any other hardware) and dynamically adjusting its use of these resources based on the resource requirements of the scheduled task workload. The ultimate scalability and hardware capabilities of the solution is dictated by the characteristics of the tasks themselves with respect to these. For example, for the use case described here these would mean the hardware and scalability capabilities of LAMMMPS with a further multiplicative factor coming from the library for the number of tasks running simultaneously. There is, unsurprisingly, room for further improvement and development, in particular related to error handling and limitations related to the Python GIL.
References jobqueue features repository, https://github.com/E-CAM/jobqueue_features  Dask documentation, https://dask.org.  Dask-Jobqueue documentation, https://jobqueue.dask.org/.  A. O. Cais, D. Swenson, M. Uchronski and A. Wlodarczyk. (2019, Augoust 14). “Task Scheduling Library for Optimising Time-Scale Molecular Dynamics Simulations,” Zenodo. http://doi.org/10.5281/zenodo.3527643  Krause, D. and Thörnig, P.: JURECA: Modular supercomputer at Jülich Supercomputing Centre, http://juser.fz-juelich.de/record/850758 (2016)
We would use the present module to avoid topology violation in an entangled polymer system. To preserve topology in a system of entangled polymers we need to determine the minimal distance between two bonds. Once is done we can apply either a soft potential either a hard potential to avoid a crossing of the two bonds. Here, we propose to determine the minimal distance between two segments with the help of the Karush-Kuhn-Tucker conditions.
This module is a part of an E-CAM pilot project at the ENS Lyon, focused on the implementation of contact joint to resolve excluded volume constraints
A detailed derivation of the minimal distance between two segments using the Karush-Kuhn-Tucker conditions is available at https://gitlab.e-cam2020.eu:10443/carrivain/mindist2segments_kkt/-/blob/master/minDist2segments_KKT.pdf
This module is used by other ongoing work, such as module velocities_resolve_EV, that resolves the excluded volume constraint with a velocity formulation.
The source code and more information can be found at minDist2segments_KKT GitLab repository.
A successful collaboration between the EU H2020 E-CAM and MaX Centres of Excellence, and the Swiss NCCR MARVEL
In a recent paper, researchers from the Centres of Excellence E-CAM and MaX, and the centre for Computational Design and Discovery of Novel Materials NCCR MARVEL, have proposed a new procedure for automatically generating Maximally-Localised Wannier functions (MLWFs) for high-throughput frameworks. The methodology and associated software can be used for hitherto difficult cases of entangled bands, and allows the electronic properties of a wide variety of materials to be obtained starting only from the specification of the initial crystal structure, including insulators, semiconductors and metals. Industrial applications that this work will facilitate include the development of novel superconductors, multiferroics, topological insulators, as well as more traditional electronic applications.
Predicting the properties of complex materials generally entails the use of methods that facilitate coarse grained perspectives more suitable for large scale modelling, and ultimately device design and manufacture. When a quantum level of description of a modular-like system is required, this can often be facilitated by expressing the Hamiltonian in terms of a localised, real-space basis set, enabling it to be partitioned without ambiguity into sub-matrices that correspond to the individual subsystems. Maximally-localised Wannier functions (MLWFs) are particularly suitable in this context. However, until now generating MLWFs has been difficult to exploit in high-throughput design of materials, without the specification by users of a set of initial guesses for the MLWFs, typically trial functions localised in real space, based on their experience and chemical intuition.
E-CAM scientist Valerio Vitale and co-authors from the partner H2020 Centre of Excellence MAX and the Swiss based NCCR MARVEL  in a recent article look afresh at this problem in the context of an algorithm by Damle et al, known as the selected columns of the density matrix (SCDM) method, as a method to provide automatically initial guesses for the MLWF search, to compute a set of localized orbitals associated with the Kohn–Sham subspace for insulating systems. This has shown great promise in avoiding the need for user intervention in obtaining MLWFs and is robust, being based on standard linear-algebra routines rather than on iterative minimisation. In particular, Vitale et al. developed a fully-automated protocol based on the SCDM algorithm in which the three remaining free parameters (two from the SCDM method, plus the choice of the target dimensionality for the disentangled subspace) are determined automatically, making it thus parameter-free even in the case of entangled bands. The work systematically compares the accuracy and ease of use of standard methods to generate localised basis sets as (a) MLWFs; (b) MLWFs combined with SCDM’s and (c) using solely SCDM’s; and applies this multifaceted perspective to hundreds of materials including insulators, semiconductors and metals.
This is significant because it greatly expands the scope of materials for which MLWFs can be generated in high throughput studies and has the potential to accelerate the design and discovery of materials with tailored properties using first-principles high-throughput (HT) calculations, and facilitate advanced industrial applications. Industrial applications that this work will facilitate include the development of novel superconductors, multiferroics, topological insulators, as well as more traditional electronic applications.
In SCDM Wannier Functions, E-CAM has implemented the SCDM algorithm in the pw2wannier90 interface code between the Quantum ESPRESSO software and the Wannier90 code. This was done in the context of an E-CAM pilot project at the University of Cambridge. Researchers have then used this implementation as the basis for a complete computational workflow for obtaining MLWFs and electronic properties based on Wannier interpolation of the Brillouin zone, starting only from the specification of the initial crystal structure. The workflow was implemented within the AiiDA materials informatics platform (from the NCCR MARVEL and the MaX CoE) , and used to perform a HT study on a dataset of 200 materials.
See the Materials Cloud Archive entry. A downloadable virtual machine is provided that allows to reproduce the results of the associated paper and also to run new calculations for different materials, including all first-principles and atomistic simulations and the computational workflows.
Bibliography Automated high-throughput Wannierisation, Valerio Vitale, Giovanni Pizzi, Antimo Marrazzo, Jonathan R. Yates, Nicola Marzari and Arash A. Mostofi, Nature Computational Materials (2020)6:66 ; https://doi.org/10.1038/s41524-020-0312-y  https://www.e-cam2020.eu/  http://www.max-centre.eu/  https://nccr-marvel.ch/  Compressed Representation of Kohn−Sham Orbitals via Selected Columns of the Density Matrix , Anil Damle, Lin Lin, and Lexing Ying, J. Chem. Theory Comput. 2015, 11, 1463−1469 https://pubs.acs.org/doi/10.1021/ct500985f
HPC Carpentry – a way forward
E-CAM is helping to organise a session on HPC Carpentry during CarpentryCon @ Home that aims to foster a scalable HPC training model. Join our Software Manager Alan O’Cais on Monday 20 July, at 9 am and at 5pm CEST. More details about the session at https://2020.carpentrycon.org/schedule/#session-33. View the session’s Etherpad and sign up at https://pad.carpentries.org/cchome-hpc-carpentry
Session “HPC Carpentry – a way forward” at the CarpentryCon @ Home
Session 1: July 20, 2020 at 07h00 UTC (9h00 CEST)
Session 2: July 20, 2020 at 15h00 UTC (17h00 CEST)
- Alan O’Cais, E-CAM Software Manager, Jülich Supercomputing Centre (JSC), Germany
- Peter Steinbach, Helmholtz AI Consultants Team Lead for Matter Research, Helmholtz-Zentrum Dresden-Rossendorf, Germany
More information about the session and sign up:
The new CECAM webinar series entitled “The importance of being H.P.C. Earnest”, will focus on of HPC as an enabler of leading-edge simulation and modelling, and on the science made possible by combining state-of-the-art methods with optimal exploitation of supercomputing resources.
A series of 5 CECAM webinars will be held every Thursday 15:00-17:00 (CEST) and broadcasted live on the CECAM YouTube Channel, starting on June 18 2020.
Different experts, who are also key players in projects targeting software development for high-end computational facilities, such as the European Centers of Excellence for Computing Applications and analogous initiatives based in the United States of America, will be present for this occasion.
The E-CAM Centre of Excellence will be featured on Thursday 2 July 2020 by Prof. Ignacio Pagonabarraga, CECAM Director and Technical Manager of E-CAM.
The full programme for the webinar series is the following:
Chapter 1: Thursday, 18 June 2020
Nicola Marzari – EPFL
Claudia Filippi – University of Twente
Anthony Scemama – University of Toulouse III
Giulia Galli – University Of Chicago And Argonne National Laboratory
Chapter 2 : Thursday, 25 June 2020
Erik Lindahl – Stockholm University
Jesus Labarta – Barcelona Supercomputing Center
Paul Kent – Oak Ridge National Laboratory
Chapter 3 : Thursday, 2 July 2020
Cecilia Clementi – Freie Universität Berlin
Ignacio Pagonabarraga – CECAM
Peter Coveney – University College London and University of Amsterdam
Chapter 4 : Thursday, 9 July 2020
Edouard Audit – CEA
Elisa Molinari – University of Modena
Gianluca Palermo – Politecnico di Milano
Chapter 5: Thursday 16 July 2020
Steven G. Louie – University of Berkeley
Claudia Draxl – Humboldt University Berlin
HPC facilities are a major capital investment and often run close to capacity. Improving the efficiency of application software running on these facilities either speeds up time to solution or allows for larger, more challenging problems to be solved. The Performance Optimisation and Productivity (POP) Centre of Excellence exists to help academic and industry groups identify how their software can be improved, free of charge. Funded by the EU under the Horizon 2020 Research and Innovation Programme, POP puts the world-class HPC expertise of eight commercial and academic partners at the disposal of European Scientists and Industry.
Collaborations with the POP CoE
Given that POP is home to a large set of performance experts, E-CAM has collaborated with them on (to date) two applications that are of particular interest to E-CAM with respect to extreme scalability: ESPResSo++ and PaPIM. We have also benefitted from their HPC specialists in one of our Extended Software Development Workshops organized by the Electronic Structure Library initiative (ESL), where POP’s experts provided a 1.5 day Tutorial on advanced performance and scalability profiling of the ESL libraries.
Successful collaboration with POP: Optimization of PaPIM
POP carried out a study of PaPIM which resulted in a 10 page report on its performance, highlighting issues in the code and proposing remedies. For example, the report showed that load imbalance issues in the expensive part of the application was mainly related to an uneven spread of the sample groups among the MPI tasks. Of more interest was the communication pattern, where the POP analysis showed that replacing a number of successive collective communications with a single collective of a derived data type could lead to a 4.7 x improvement in communication performance.
How it works
A simple request form should be completed at https://pop-coe.eu/request-service-form. One of their technical experts will be in touch to obtain the details.
Briefly, POP services involve the following steps.
- The first step is to profile the application behaviour using suitable parallel profiling tools, e.g. Extrae or Scalasca. This step creates trace files which require analysis by POP experts. This is typically done on the user’s machines. However, if this is not an option for a user, POP can collect performance data on one of their HPC machines. This task can be done either by POP experts or by users with POP support.
- The results from the analysis of the trace files are presented to the user, explaining the performance issues with the code and recommendations for performance improvements. Experience shows that it is often difficult to build a quantitative picture of parallel application behaviour. One of the strengths of POP is their set of metrics, which provide a standard, objective way to characterise different aspects of the performance of parallel codes.
- POP performance assessment can be followed up by further work, again completely free to the user, to demonstrate how to implement these improvements.
A feature that is particularly useful when dealing with industrial partnerships, is that POP services don’t require access to the source code – they can work with executables. And if needed, non-disclosure agreements can be signed.
 PaPIM is a code for computing time-dependent correlation functions and sampling of the phase space. It samples the phase space either classically or quantum mechanically. Documentation available here.
This module is the first in a sequence that will form the overall capabilities of the E-CAM High Throughout Computing (HTC) library. In particular this module deals with creating a set of decorators to wrap around the Dask-Jobqueue Python library, which aspires to make the development time cost of leveraging it lower for our use cases.
The initial motivation for this library is driven by the ensemble-type calculations that are required in many scientific fields, and in particular in the materials science domain in which the E-CAM Centre of Excellence operates.
One specific application for this module is the study of “rare events” in theoretical and computational chemistry, a particularly relevant topic for E-CAM . Many problems in biological chemistry, materials science, and other fields involve events that only spontaneously occur after a millisecond or longer (for example, biomolecular conformational changes, or nucleation processes). That means that around 1012 time steps would be needed to see a single millisecond-scale event.
Modern supercomputers are beginning to make it possible to obtain trajectories long enough to observe some of these processes, but to fully characterize a transition with proper statistics, many examples are needed. In order to obtain many examples the same application must be run many thousands of times with varying inputs. To manage this kind of computation a task scheduling high throughput computing (HTC) library is needed. The main elements of the mentioned scheduling library are: task definition, task scheduling and task execution.
While traditionally an HTC workload is looked down upon in the HPC space, the scientific use case for extreme-scale resources exists and algorithms that require a coordinated approach make efficient libraries that implement this approach increasingly important in the HPC space. The 5 Petaflop booster technology of JURECA is an interesting concept with respect to this approach since the offloading approach of heavy computation marries perfectly to the concept outlined here.