Quantum Monte Carlo (QMC) methods are a class of ab initio, stochastic techniques for the study of quantum systems. While QMC simulations are computationally expensive, they have the advantage of being accurate, fully ab initio and scalable to a large number of cores with limited memory requirements.

These features make QMC methods a valuable tool to assess the accuracy of DFT computations, which are widely used in the fields of condensed matter physics, quantum chemistry and material science.

QMCPack is a free package for QMC simulations of electronic structure developed in several national labs in the US. This package is written in object oriented C++, offers a great flexibility in the choice of systems, trial wave functions and QMC methods and supports massive parallelism and the usage of GPUs.

Trial wave functions for electronic QMC computations commonly require the use of  single electrons orbitals, typically computed by DFT. The aim of the E-CAM pilot project described here is to build interfaces between QMCPack and other softwares for electronic structure computations, e.g. the DFT code Quantum Espresso.

These interfaces are used to manage the orbital reading or their DFT generation within QMCPack, to establish an automated, black box workflow for QMC computations. QMC simulation can for example be used in the benchmark and validation of DFT calculations: such a procedure can be employed in the study of several physical systems of interest in condensed matter physics, chemistry or material science, with application in the industry, e.g. in the study of metal-ion or water-carbon interfaces.

The following modules have been built as part of this pilot project:

• QMCQEPack, that provides the files to download and  properly patch Quantum Espresso 5.3 to build the libpwinterface.so library; this library is required to use the module ESPWSCFInterface to generate single particle orbitals during a QMCPack computation using Quantum Espresso.

• ESInterfaceBase that provides a base class for a general interface to generate single particle orbitals to be used in QMC simulations in QMCPack; implementations of specific interfaces as derived classes of ESInterfaceBase are available as the separate modules as follows:
  • ESHDF5Interface
  • ESPWSCFInterface

The documentation about interfaces in QMCPack, can be seen in the QMCPack user manual at https://github.com/michruggeri/qmcpack/blob/f88a419ad1a24c68b2fdc345ad141e05ed0ab178/manual/interfaces.tex