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Adding the molecular dynamics functionality to the quantum Monte Carlo code CASINO

This was the second Distributed Computational Science and Engineering (dCSE) project concerning the Quantum Monte Carlo (QMC) code CASINO, which builds on the success of the first project. CASINO may be used for performing quantum Monte Carlo (QMC) electronic structure calculations for finite and periodic systems and is developed at the Cavendish Laboratory, University of Cambridge.

The overall aim of this work is to develop functionality in CASINO to enable coupled Diffusion Monte Carlo (DMC) and Density Functional Theory (DFT) molecular dynamics simulations. This will be achieved as follows:

  • A scalable interface will be developed between the PWscf code (which is part of Quantum ESPRESSO). (Interfaces between CASINO and a number of DFT codes already exist.)
  • CASINO will be modified with an algorithm based on re-weighting, level-cross checking and periodic Jastrow recomputation.
  • An interface will be written for the data communication and transfer between PWscf and CASINO. In particular the I/O and FFT parts of the coupling will be developed for efficiency.

The achievements of the project are summarised below:

  • A Direct interface with CASINO was developed for the PWscf DFT code.
  • The interface between PWscf and CASINO is now provided through a file with a standard format containing the geometry, basis set, and orbital coefficients. CASINO support is implemented by three routines in the PW directory: pw2casino.f90 (the main routine), pw2casino_write.f90 (which writes the CASINO file in various formats) and pw2blip.f90 (which does the plane-wave to blip conversion, if requested).
  • The parallel efficiency of the DMC algorithm was optimised by improving the allocation and re-distribution of walkers (i.e. the list of current electron positions for the configuration, along with various associated quantities related to the energy and wave function) between the cores. This was achieved by implementing asynchronous communication to overlap with computation.
  • The work on parallel efficiency significantly improved the behaviour of CASINO. For a fixed target of 100 per core for a total walker population, linear weak scalability has been demonstrated for over 80,000 cores on the Jaguar Cray XT5 machine (1.75 petaFLOPS) at Oak Ridge National Laboratory. Furthermore, with 82,944 cores on Jaguar, CASINO version 2.7 is more than 30% faster and the total cost of re-distributing walkers (including the DMC equilibration) was reduced from 412 seconds to 1 second. This means it will be possible to use CASINO on machines with in excess of 100,000 cores.
  • Further minor modifications were also made to the CASINO distribution including the following: multiple pseudopotentials for elements with the same atomic number, support for running multiple jobs simultaneously, faster partial-ranking algorithm, interface support for CRYSTAL09 and a major revision of the user manual.
  • This work has been introduced within the main CASINO (version 2.10) and PWscf (version 4.3) code bases.

Please see PDF for a report which summarises this project.