# Changelog

## Berkeley 2.1 (Jul/2019)

BerkeleyGW 2.1 is the first version based on a new and more modern coding infrastructure, with better support for new compilers and improved consistency checks. For the end users, the main noticeable features include increased performance and bug fixes for I/O operations involving the new HDF5 file format and for the subspace code. We also improved considerably the documentation of the code, with a new and expanded user manual.

Some highlights and features for the end user:

• New user manual for the code, which comes bundled with the code, and which is also available online: http://manual.berkeleygw.org

• New wrapper for the StochasticGW code.

• Bug fixes when writing HDF5 wavefunctions in parallel which could cause the code to hang (relevant to the ParaBands code).

• Improved error checking for operations involving HDF5 files.

• Improved support for compilers, including PGI and NAG.

• Improved performance and stability for building BerkeleyGW in parallel with new dependency system.

• Improved performance of HDF5 routines for the subspace code and for reading chimat.h5 files.

## Berkeley 2.0 (May/2018)

BerkeleyGW 2.0 represents the culmination of nearly two years of development effort, and this release contains a number of important new features and capabilities including:

Main new features:

1. Initial release of ParaBands: a new tool for efficiently generating wave-function files including many empty orbitals required for BerkeleyGW calculations.

2. Full BSE calculations that do not employ the Tamm-Dancoff approximation.

3. Improved algorithms for k-point sampling in 2D, which include the newly proposed nonuniform neck subsampling (NNS) and the cluster sampling interpolation (CSI) algorithms.

4. Accelerated full-frequency GW calculations through the use of a low-rank subspace approximation for expressing the dielectric matrix. In fact, large-scale full-frequency GW calculations are now faster than calculations using plasmon-pole models!

5. Significant performance improvements throughout, but particularly in the calculation of the full-frequency dielectric matrix and evaluation of the full-frequency Sigma operator. Continued optimizations were made throughout the package for multi- and many-core architectures including Intel Xeon-Phi, which allows BerkeleyGW to scale half a million cores on Cori 2 for large-scale calculations!

6. Improved user and developer documentation, as well as a new quick reference guide (see the link on the top of the page).

## Berkeley 1.2 (Aug/2016)

F. H. da Jornada, J. Deslippe, D. Vigil-Fowler, J. I. Mustafa, T. Rangel, F. Bruneval, F. Liu, D. Y. Qiu, D. A. Strubbe, G. Samsonidze, J. Lischner.

Features marked with [*] change the default behavior of the code relative to BGW-1.1 and may cause a small change in the numbers produced by the code.

### New features

1. Added new methods to deal with the frequency-dependence of the polarizability:

2. Added Godby-Needs (GN) plasmon-pole model.

3. Added spectral method for real-axis (RA) full-frequency (FF) calculations. The spectral method allows one to compute the dielectric matrix much faster for many frequency points on the real axis. The old method for RA-FF calculations is now refered to as the Adler-Wiser method. Still, RA is no longer the default FF method (see next item).
4. Added Contour-deformation (CD) full-frequency (FF) formalism. The CD is the recommended (and default) scheme for calculations performing an explicit evaluation of the dynamical effects of the dielectric matrix without plasmon-pole models. It requires the evaluation of the dielectric matrix on both real and imaginary frequencies, but with typically a much smaller number of frequencies. The previous formalism is now refered to as real-axis (RA) full-frequency (FF) formalism. CD is now the default method for FF calculations.

5. Released non-linear optics post-processing utility: This codes allows one to compute the inter-exciton transitions in non-linear-optics experiments.

6. Improved usability in the code output and added dynamic verbosity switch: BerkeleyGW has now a much simpler and neater output. The code gives time estimates for most tasks it performs -- at least for those that are more time-consuming. There is also a run-time flag supported by all codes to switch the amount of verbosity the code produces. The old compilation flag -DVERBOSE is now deprecated.

7. Improved usability in the code input: Several input parameters, such as band occupations, are automatically detected. If not set, parameters such as the cutoff for the bare and screened Coulomb potential are also read from the wave function and dielectric matrix, respectively.

8. Automatic solution of Dyson's equation in FF calculations: BerkeleyGW now automatically solves Dyson's equation for FF calculations. It uses a varying number of frequencies to find the graphical solution for the quasiparticle energies, and perform extrapolation (with appropriate warning messages) if no intersection could be found. The code will also output the files eqp0.dat and eqp1.dat directly, containing the off-shell and on-shell quasiparticle energies.

9. Added ABINIT wrapper: It is possible to interface the ABINIT code with BerkeleyGW. For now, the wrapper can only output complex-valued wavefunctions, even for systems with inversion symmetry.

### Improvements

1. Added support for Xeon-Phi Knight's Landing (KNL)-based systems.

2. Improved OpenMP scaling throughout. Using OpenMP is now recommended for large-scale computations.

3. Added new and more performant HDF5 file formats for epsmat and bsemat matrices. This is recommend as default for all builds. Note that files are generally not compatible with BerkeleyGW 1.1 and earlier. Binary epsmat files can be converted to the HDF5 format with the epsmat_old2hdf5 utility. Older epsmat.h5 files from BerkeleyGW-1.1-beta2 can be converted to the current format with the epsmat_hdf5_upgrade utility.

4. Added parallelization over frequencies in the epsilon code. This allows for better scalability in full-frequency calculations. This scheme is supported in the old Adler-Wiser and in the new Contour-Deformation formalisms.

5. Improved the Monte-Carlo average scheme used to compute the Coulomb potential. [*] For bulk systems, the new default scheme is to compute the average of Coulomb potential v(q+G) for all q-points and G-vectors. We use a hybrid scheme to make this evaluation faster. While the code is slightly slower for small calculations, results should converge much faster with k-point sampling. One can still use the old defaults with the input option cell_average_cutoff 1.0d-12. Note that no change was performed for 2D, 1D and 0D systems, or 3D metals.

6. Rewrote k-point interpolation engine. [*] The previous k-point interpolation scheme used in the haydock, absorption, and inteqp codes used a greedy algorithm to search for the closest coarse-grid points around each fine-grid points, which lead to discontinuities of the interpolands. The new interpolation engine uses QHull, which is bundled with BerkeleyGW, to perform a Delaunay tessellation of the coarse-grid k-points. The new interpolation scheme is much more robust. You can use the old interpolation scheme with the greedy_interpolation flag.

7. Generalized scheme for k-point interpolation. The k-point interpolation is used in the haydock, absorption, and inteqp codes. The new scheme is useful for metals and systems with valence-conduction band character mixing in the BSE.

### Misc

1. Changed logic to deal with invalid GPP frequency modes. [*] When the code performs a HL-GPP or a GN-GPP calculation and finds an invalid mode with frequency with $\omega_{G,G'}^2 < 0$, the code will now treat that mode within the static COHSEX approximation (i.e., move frequency to infinity). The previous behavior was to "find" a purely complex mode frequency and relax the causality constraint. One can switch which strategy to use with the invalid_gpp_mode input flag.

2. Many bug fixes and performance improvement.