Quantum Espresso (opEn Source Package for Research in Electronic Structure, Simulation, and Optimization) is a freely available (GPL) suite of software for perfoming materials science using Density Function Theory (DFT). Applications include ground state calculations, structural optimisation, ab initio molecular dynamics, linear response (phonons) and spectroscopy. It uses a plane wave basis, in common with similar packages like CASTEP and CPMD, and implements a range of pseudopotentials and exchange correlation functionals, including Hartree-Fock and hybrid functionals (PBE0, B3LYP, HSE). It is developed by an international collaboration of the DEMOCRITOS National Simulation Center (Trieste) with several other materials science centers in Europe and the USA.
Unlike other packages, Quantum Espresso is not a monolithic single executable, but rather a set of independent executables, each of which perform a particular function. For this project the executables of interest were PW.x (Electronic and Ionic Structure, including MD), and PH.x (Phonons using Density Functional Perturbation Theory). Quantum Espresso is written in Fortan 90, consisting of over 300,000 lines of code in nearly 1000 source files. In addition to the source for each executable, there is a common Modules directory containing code shared by all the executables. This includes common functionality such as I/O, parallelisation, data types, as well as the FFT. A number of external libraries are required including an FFT library (e.g. FFTW3) as well as BLAS, LAPACK and ScaLAPACK for linear algebra operations.