##### Copyright (C) 2001-2003 ABINIT group (XG,DCA)
This file is distributed under the terms of the GNU General Public License, see ~abinit/COPYING or http://www.gnu.org/copyleft/gpl.txt .
For the initials of contributors, see ~abinit/doc/developers/contributors .

    Features_v4.1

    Description of the major features of the version 4.1
    of the ABINIT code.

    Content :
    0. Related documentation.
    1. Available physical properties.
    2. Speed and memory.
    3. The user's point of view.
    4. The programmer's point of view.

    ---------------------------

    0. Related documentation
    ------------------------

    The reader might consult the latest version of the 'context' file
    for the description of the ABINIT project and its history.
    The latest version of the 'planning' file will give an idea of
    future developments.
    The different versions of the 'release_notes' files will allow to
    see the actual development of the project since version 1.5,
    released in August 1998.
    Then, there are also the 'new_user_guide' and 'abinis_help' files,
    for accurate descriptions of the code and its use.

    1. Available physical properties
    ________________________________

    A. Computation of the total energy of an assembly of nuclei and electrons
    placed in a repeated cell.

    A.1. The computation is done using plane waves and pseudopotentials.

    A.2. The total energy computation is done according to the Density Functional
    Theory. Most of the important local approximations (LDA) are
    available, including the Perdew-Zunger one. Two different local spin density
    (LSD) are available, including the Perdew Wang 92, and one due to M. Teter.
    The Perdew-Burke-Ernzerhof,  the revPBE, the RPBE
    and the HCTH GGAs (spin unpolarized as well as polarized)
    are also available.

    A.3. Self-consistent calculations will generate the DFT ground-state,
    with associated energy and density. Afterwards, a non-self-consistent
    calculation might generate eigenenergies at a large number
    of k-points, for band structures. The electronic Density-Of-States
    can be computed either within the tetrahedron method, or using a smearing
    technique.

    A.4. The program admits many different types of pseudopotentials.
    There are two complete sets of pseudopotentials available for
    the whole periodic table, one of the Troullier-Martins type,
    one of the Goedecker type (this one include the spin-orbit coupling).
    Four codes are available to generate
    new pseudopotentials when needed. Two of them are able to generate
    pseudopotentials with a core hole, in order to compute core-level shifts.
    Two of them are able to generate GGA pseudopotentials.
    No ultra-soft pseudopotential can be used.
    Pseudopotentials can be mixed, to generate "alchemical" pseudoatoms.

    A.5. Metallic as well as insulating systems can be treated. Schemes for
    determination of the occupation number include the Fermi broadening,
    the Gaussian broadening, the Gaussian-Hermite broadening, as well as
    the modifications proposed by Marzari. Finite temperatures
    can also be treated using a smearing scheme (Verstraete scheme).

    A.6. The cell may be orthogonal or non-orthogonal.
    Any kind of symmetries and corresponding sets of k-point can be input,
    and taken into account in the computation.

    A.7. The electronic system may be computed in the
    spin-unpolarized or spin-polarized
    case, with the possibility to impose occupation numbers of majority and
    minority spins, and the spins of the starting configuration.
    A specific option for efficient treatment of
    anti-ferromagnetism (Shubnikov groups) is available.
    The treatment of non-collinear magnetism is available, but only for
    the total energy (no forces, stresses, reponse-functions ...).
    The total magnetic moment of the unit cell can be constrained.

    A.8. The total energy, forces, stresses, and electronic structure
    can be provided with the spin-orbit coupling included.

    A.9. The decomposition of energy in its different component (local potential,
    XC, hartree ...) is provided.

    A.10. Inner electronic eigenvalues can be computed thanks to the
    minimisation of the residual with respect to a target energy value.

    B. Derivatives of the total energy and eigenenergies

    B.1. Hellman-Feynman forces are computed from an analytical formula,
    and corresponds exactly to the limit of finite differences of energy
    for infinitesimally small atomic displacements when the ground-state
    calculation is at convergence. This feature is available for all the
    cases where the total energy can be computed (except non-collinear
    magnetism).
    A correction for non-converged
    cases allows to get accurate forces with less converged wavefunctions
    than without it. The decomposition of the forces in their different
    components can be provided.

    B.2. Stress can also be computed. This feature is available for all
    the cases where the total energy can be computed, except
    non-collinear magnetism.
    The decomposition of the stresses in their different
    components can be provided. A smearing scheme applied to the kinetic
    energy allows to get smooth energy curves as a function of
    lattice parameters and angles.
    Alternatively, a facility for correcting the computed stress
    by the Pulay stress is provided.

    B.3. The polarization can be computed within the Berry phase
    formulation. This feature is available for insulators,
    magnetic or non-magnetic, but not yet when spin-orbit splitting
    is present. The localisation tensor can also be computed.

    B.4. Accurate responses to atomic displacements and
    homogeneous electric fields
    are available, and allows to compute the interatomic force constants,
    the Born effective charges, the dielectric constant, the phonon band
    structure. Symmetry characters of the phonons at Gamma
    are computed. Thermodynamical properties, like the free energy,
    the heat capacity or the entropy, can also be computed, in the
    quasi-harmonic approximation. Available for the GGAs (except the HCTH
    one), while for spin-orbit, only responses to atomic displacements are
    available.

    B.5. Approximate or accurate susceptibility matrix
    and dielectric matrix can be computed, thanks to a sum over states.

    B.6. Derivatives of the electronic eigenenergies with respect
    to the wavevector can be computed analytically.

    B.7. Optical conductivity can be computed, thanks to the Kubo-Greenwood
    formula.

    B.8. The band-by-band decomposition of the Born effective charges,
    and the localisation tensor is available.

    B.9. Selected bon-linear response coefficients can be computed thanks to the
    2n+1 theorem of perturbation theory. At present :
    - the electro-optic coefficients
    - the Raman cross sections

    C. Excited states

    C.1. Computation of ionisation energies (N -> N-1 electrons)
    and affinities (N -> N+1 electrons) in the GW approximation.

    C.2. Excited states of atoms and molecules (spin-singlet as well
    as spin-triplet) can be computed within TDDFT. Oscillator strengths
    are available.

    D. Displacement of atoms, and changes of cell parameters.

    D.1. Different algorithms (Broyden; modified Broyden; Verlet with sudden
    stop of atoms) allows to find the equilibrium configuration
    of the nuclei, for which the forces vanish. The cell parameters
    can also be optimized concurently with the atomic positions.
    Specified lattice parameters, or angles, or atomic positions,
    can be kept fixed if needed.

    D.2. Two molecular dynamics algorithm (Numerov or Verlet)
    allow to perform simulations in real
    (simulated) time. The displacement of atoms may be computed according to
    Newton's law, or by adding a friction force to it.
    Nose-Hoover thermostat is available with Verlet algorithm.
    Langevin dynamics is also available.

    D.3. The code can provide an automatic analysis of bond lengths and angles,
    and the atomic coordinates in a format suitable for vizualisation with XMOL.

    E. Analysis and graphical tools.

    E.1. A post-processor, called "cut3d", is available to analyse density,
    potential and wavefunction files. It is able to change the format of these
    files, to extract the data on a 2D plane, or along a 1D line.
    It can perform the Hirshfeld computation of atomic charges.
    It can analyse the charge contained in an atomic sphere,
    and determine the angular momentum projected charge (l=0 to 4)
    contained in that sphere.

    E.2. Another post-processor, called "aim", is available to
    perform the Bader "Atom-In-Molecule" analysis of the density.

    E.3. A special part in the tutorial (see later) indicates how
    to generate properly formatted data for the visualisation of :
    - the band structure (visualisation thanks to XMGR)
    - total energies vs different parameters (also using XMGR)
    - the charge density (3D isosurfaces)
     (the cut3d postprocessor must be used, followed by matlab)
    The cut3d postprocessor also allows to prepare 2D charge density plots.

    E.4. The post-processor band2eps allows to draw phonon dispersion
    curves automatically, in a file written in Encapsulated PostScript (eps).
    Moreover, a color code allows to emphasize the contribution of individual
    atoms to the corresponding eigenvector (at most three types of atoms).

    2. Speed and memory.
    ____________________

    A. Speed in the sequential version

    A.1. Depending on the number of atoms, there are two regimes in the
    code : at low number of atoms and electrons,
    the CPU time is dominated by Fast Fourier
    Transforms with an average scaling O(N^2 log N) where N is some
    number characteristics of the size of the system (atoms, electrons);
    at large number of atoms and electrons, the CPU time is dominated
    by non-local operator aplication and orthogonalisation, with
    an average scaling O(N^3).

    A.2. The complex-to-complex Fast Fourier Transform routine for application
    of the Hamiltonian has been highly optimized, and take into account
    the fact that the wavefunction do not fill the reciprocal space FFT box.
    Library FFTs are also available, but they are found to be slower than the
    present FFT routine, developed starting from a routine
    provided by S. Goedecker.
    A real-to-complex FFT is used for treating potential and densities
    of the ground state, since they are real.
    For selected k-points, invariant under time-reversal symmetry,
    ( (0 0 0), (1/2 0 0), (0 1/2 0), (0 0 1/2), (1/2 1/2 0) ... (1/2 1/2 1/2) ),
    the number of planewave explicitly treated is divided by two. A
    real-to-complex FFT is used then.

    A.3. The non-local potential is applied in reciprocal space. It has been
    optimized carefully, although there is still some speed-up to be
    coded when the k-point is invariant under time-reversal.
    The orthogonalisation procedure can be done twice per loop or only once.

    A.4. At the level of the generation of electronic eigenfunctions, an efficient
    band-by-band preconditioned conjugate-gradient algorithm is used,
    in its non-self-consistent version.

    A.5. At the level of the self-consistency loop, an efficient
    potential-based preconditioned conjugate-gradient algorithm is used.
    Simple mixing is also available, as well as the Anderson algorithm.
    Preconditioning of this algorithm is achieved through a model
    dielectric function, or through an approximate dielectric matrix.

    B. Speed in the parallel version

    B.1. For ground-state calculations, the code has been parallelized
    on the k-points, on the spins, on the bands,
    and on the FFT grid and plane wave coefficients.
    For the k-point and spin parallelisations (using MPI), the communication
    load is generally
    very small. This allows it to be used on a cluster of workstations.
    However, the number of nodes that can be used in parallel might
    be small, and depends strongly on the physics of the problem.
    The band parallelisation (also using MPI) can be used
    concurently with the k point and and spin parallelisation,
    but is less efficient.
    The FFT grid parallelisation (using OpenMP) works only
    for SMP machines, and is still to be
    optimized. Alternatively, a MPI version is under development.

    B.2. For response calculations, the code has been parallelized
    on k-points, spins, and bands (MPI), as well as
    on the FFT grid and plane wave coefficients (OpenMP).
    For the k-points, spins and bands parallelisation,
    the communication load is rather
    small also, and, unlike for the GS calculations, the number
    of nodes that can be used in parallel will be large,
    nearly independently of the physics of the problem.
    The FFT grid parallelisation (using OpenMP) works only
    for SMP machines, and is still to be
    optimized.

    B.3. A careful study of the speed-up should still be done,
    in both the GS and RF cases.

    C. Memory.

    C.1. The requirements of the different conjugate gradient algorithms
    on memory are relatively low, especially when the number of atoms
    is large. Optionally, it is even possible to use disk space
    to save memory, at the expense of computing time.
    In particular, when the number of k points is large, they can
    be stored in memory one at a time. Phase factors in the application of
    the non-local operator can also be recomputed at each application,
    in order to save memory. For k-points that are invariant under
    time-reversal symmetry, the storage required for wavefunctions
    is half the storage for other k-point.

    3. The user's point of view.
    ____________________________

    A. The Web site.

    A.1. A Web site can be accessed. The complete sources (and all the tests)
    of the ABINIT package are available there.
    Executables for many different platforms are also available,
    in specific packages that also include
    the 'Infos' directory are also available.
    Installation notes, current features of ABINIT,
    the tutorial, on-line help can be vizualized directly from the web.

    A.2. Also available from the Web site :
    - the pseudopotentials
    - some utilities (including cut3d, a density analyser),
    - four mailing lists
    (one for "official announcements", one for the developpers,
     one "forum mailing list" , one for the advisory commitee).
    - the ABINIT bibliography database, that contains references of papers
    in which ABINIT or one of its predecessors have been used.
    - the FAQ (frequently asked questions) database

    B. Portability.

    B.1. The ABINIT package has been installed successfully on the
    following different platforms :

    - PC/Linux based on PPro, PII, PIII, PIV processors, with
     pghpf compiler, Intel compiler, fujitsu compiler, NAG compiler.
    - PC under Windows
    - HP/SPP1600, HP/S-class, HP/N-class based on the HP 7200, 8000
     and 8500 processors.
    - DEC alpha workstations under OSF, based on EV56, EV6, EV67 or EV68.
    - DEC alpha workstations under Linux, based on EV56 .
    - IBM RS6000 (models : 590, 3CT, nighthawk) based on Power 2 and 3+ processors.
    - SGI Origin 2000
    - CRAY T3E
    - FUJITSU VPP-700
    - Sun ultrasparc II
    - NEC
    - HITACHI SR8000
    - Mac OS X

    B.2. In particular, the parallel version is available on clusters of
    Intel/Linux, DEC or IBM workstations, as well as on
    CRAY T3E, SGI Origin 2000, HP/SPP1600, HP/S-class, HP/N-class,
    FUJITSU VPP-700 machines, HITACHI SR8000, IBM 44P, Sun ultrasparc.

    B.3. Installation is made thanks to a sophisticated (but robust) suite of
    makefiles and scripts, and use a file preprocessor.
    Thanks to these, all machine-dependent parameters
    are grouped in one single short file for each machine.
    The parallel and sequential
    version of the code, as well as the different versions for the different
    machines, are prepared on-the-fly, by this suite of makefiles and scripts,
    so that there is only one unique source code.

    B.4. Binaries for different machines can be managed in the
    same main directory, as they might be placed automatically in different
    sub-directories.

    C. Running jobs, input and output files.

    C.1. The input variables are gathered in one unique file,
    read by a text processing facility build in the code.
    Many defaults values are provided, so that
    the input file can be kept rather short.

    C.2. Many different stopping criteria allow the user to target the accuracy
    he or she wants to obtain.

    C.3. The outputs are provided to one main file and one auxiliary file,
    as well as different specialized files (for density, potential,
    wavefunctions, ...) .
    The main file is shorter than the auxiliary file, and well formatted,
    while all important results are gathered
    there. It can be used for archival purposes. The auxiliary (log) file
    will contain all exception messages.

    C.4. Exception handling is provided through four different types
    of messages : COMMENT, WARNING, BUG and ERROR. In each case, the
    accurate meaning of the exception is described, as well as the
    eventual action to be taken by the user.

    C.5. Statistics of foreseen memory and disk usage is printed at
    the beginning of the run. Statistics of CPU time usage
    is printed at the end of the run.

    C.6. There is a facility to stop the run in a clean way at any time.
    The user may specify a cpu time limit, after which the job must end
    smoothly.

    C.7. A status file, updated very frequently, gives an on-the-fly
    report of progress of the current run.

    C.8. The code can handle multiple datasets contained in the input
    file, where generic input variables valid for all datasets
    can be defined. These calculations for different dataset
    can be chained, so that in one run, many complex tasks can be accomplished.
    This allows easy convergence studies.

    C.9. The code can start the current run from a wavefunction input file
    generated in a previous run. This allows to cut down the number
    of iteration to self-consistency, or to perform other tasks.
    This can be done even if the previous job
    had different computational parameters, like different k points,
    different energy cut-off, spin-unpolarized or spin-polarized wavefunctions,
    scalr or spinor wavefunctions ... Of course, the restart from
    wildly different parameters will not save a lot of CPU time.

    C.10. ABINIT can produce CML (Chemical Markup Language) output files.
    It can also read the CML files it produced.

    D. Documentation.

    D.1. A new_user_guide and a few rather detailed help files
    (abinis_help, respfn_help, anaddb_help, mrgddb_help, newsp_help,
    aim_help ... ) are available.

    D.2. A tutorial is available. It starts with the computation of
    different properties of the H2 molecule, describes convergence studies,
    then focuses on bulk Silicon, bulk Al and Al surface. Finally,
    it describe the computation of dynamical and dielectric
    properties of AlAs.

    D.3. Many test cases are provided, and can help the user in setting up a run.

    D.4. An ABINIT bibliography database, that contains references of papers
    in which ABINIT or one of its predecessors is available on the Web site.

    E. Generation of the k-points, geometries, and starting wavefunctions

    E.1. The code can automatically generate symmetries from the primitive cell
    and the position of atoms. In this case, it identifies
    automatically the Bravais lattice, point group and space group.
    Alternatively, it can start from the
    symmetries and generate the atomic positions from the irreducible
    set. Also, a database of the 230 spatial groups of symmetry
    is built inside ABINIT.

    E.2. Anti-ferromagnetic symmetry operations, and associated
    magnetic space groups (Shubnikov groups) can be treated with the
    same facility as usual space groups, including a database of the
    1191 Shubnikov groups.

    E.3. A geometry builder is available inside the code.
    It can take one (or two) group
    of atoms, rotate it, translate it and repeat it, then create vacancies.

    E.4. The generation of special k point sets (Monkhorst-Pack sets)
    and band structure k points can also be done directly inside ABINIT.
    A list of interesting k point sets, can be generated automatically,
    including a measure of their accuracy in term of integration
    within the Brillouin Zone.

    E.5. A utility for generating wavefunctions with new characteristic
    (cut-off, k-point) from already existing wavefunctions with
    different characteristics is available (newsp).

    F. Automatic determination of input parameters.

    F.1. Many defaults values are provided.

    F.2. The FFT grid parameters can be automatically generated
    from the cut-off energy and geometry of the system.

    F.3. The number of bands and starting occupation numbers can be automatically
    generated from the input set of atoms.

    F.4. There is a database of atomic masses.

    4. The programmer's point of view.
    __________________________________

    A. The code is distributed without charge under the
    GNU General Public Licence (GPL). This
    garantees that the future modifications of the code
    stay available to the developpers and users for free.
    This Copyright is often referred to as a "Copyleft".

    B. The code is written in clean Fortran90. Strict programming rules
    have been followed. These are documented. Comments are numerous, and
    all in english.

    C. Quick, fully automatic, testing of the code is available in the makefile,
    giving diagnostics on the validity of computed energy, forces, stresses
    and eigenvalues for five typical cases.

    D. More extensive testing is provided in five batteries of tests
    (altogether more than 350 different runs), with automatic comparison
    with results of preceding versions. A specialized diff script (called
    'fldiff') has been written in order to ease the diagnostic on the
    suite of tests. In addition to tests of the correctness of the execution,
    for the sequential or parallel version of the main code, as well as some
    utilities, there are automatic diagnostics of the speed of crucial
    routines, and the response to a load of up to 4 instances of the
    main code, running concurrently.

    E. Debugging facilities are provided inside the code, and can be directly
    accessed from the input file. The compilation can be done in both
    debugging or normal mode : the C-preprocessed files are either kept
    or removed automatically.

    F. "Design-by-contract" utility routines are present, as well as
    a flag to preprocess the sources, and eliminate the design-by-contract
    features in production mode.
    <\pre>