mirror of https://github.com/abinit/abinit.git
230 lines
12 KiB
Markdown
230 lines
12 KiB
Markdown
---
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authors: FJ, XG
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---
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# ABINIT features
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## An overview of ABINIT settings and features, for beginners and more experienced users.
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This document gives an overview of the features implemented in the ABINIT
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package, grouped in different topics, for the beginner as well as more experienced user.
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It might answer the question "How to ... with ABINIT ?", to some extent.
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It also gives a synthetic view on the needed settings.
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## 1 Foreword
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Documenting the features of a large scientific code is a complex task. The
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present list of features refers to different "topics". Each topic has a
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dedicated page, which should be quick to read, unlike the
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[[lesson:index|lessons of the tutorial]], each of which is usually at least
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one hour work. Many of the topics make the link between a physical property or
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quantity (including bibliographical references for the theory, and sometimes
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pointing to published work using this feature) and the way it is to be
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computed with ABINIT (e.g. corresponding input variable, example input files,
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and possibly tutorial(s)), and the associated post-processing tools (OPTIC, ANADDB, MULTIBINIT ...).
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This "topic"-based approach might be used by the beginner to get a broad
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overview of the capabilities of ABINIT applications as well as to the more expert user to
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quickly find the way to compute some existing quantity, or to remember which
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input variable is useful or mandatory for the calculation of a given property.
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Some topics are rather "input"-oriented (e.g. how to specify the atomic
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geometry, the occupation numbers, etc), other are more "property"-oriented
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(e.g. how to compute the elastic constants, the temperature-dependence of the
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electronic structure, etc), other are related to proper/better usage of the code.
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Care is taken not to duplicate existing more complete documentation in ABINIT,
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but to point to it if appropriate. Not all the ABINIT documentation is covered
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by the Web-accessible documents, there are still a few unlinked documents in
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the subdirectories of ~abinit/doc (work is in progress to make it all available).
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Discussions on the [ABINIT discourse forum](https://discourse.abinit.org) might also allow to get information.
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## 2 ABINIT specifications for static DFT calculations
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The topic [[topic:GSintroduction|Building an input file]] briefly explains the
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content of an ABINIT input file. The following topics go more into the
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details, however restricting to **static DFT calculations**, without doing
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anything fancy with the exchange-correlation functionals. Going beyond these
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is left for other sections (Section 3 and beyond). In
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particular, for any accurate electronic properties, e.g. correct band
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structure, optical response, or for strongly correlated electrons, please go
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beyond the present sec. 2. Also, topics related to global control parameters,
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that apply, generally speaking, to all types of calculations are explained later.
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### 2.1 Settings for atoms: cell, atoms, atomic positions, and symmetries
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1. [[topic:UnitCell|Unit cell]]
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2. [[topic:AtomTypes|Types of atoms and alchemy]]
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3. [[topic:crystal|Crystalline structure and symmetries]]
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4. [[topic:SmartSymm|Smart symmetrizer]]
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5. [[topic:AtomManipulator|Atom manipulator]] (advanced topic)
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### 2.2 Physical settings for electrons: XC functionals, atomic/pseudo potentials, metal/insulator, spin, Coulomb interaction ...
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1. [[topic:xc|Overview of available exchange and correlation functionals]]
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2. [[topic:Hybrids|Hybrid functionals]]
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3. [[topic:vdw|Van der Waals functionals]]
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4. [[topic:RPACorrEn|Correlation energy within RPA]]
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5. [[topic:PseudosPAW|Pseudopotentials and PAW atomic datasets]]
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6. [[topic:BandOcc|Bands and occupation numbers for metals and insulators]]
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7. [[topic:spinpolarisation|Spin-polarised systems and spin-orbit coupling]]
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8. [[topic:Coulomb|Coulomb interaction and charged cells]]
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### 2.3 Numerical settings for electrons: basis set, planewaves and real space sampling, Brillouin zone sampling ...
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1. [[topic:Planewaves|Planewaves and real space sampling]]
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2. [[topic:PAW|PAW special settings]]
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3. [[topic:Wavelets|Wavelets in ABINIT]]
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4. [[topic:k-points|Wavevector sampling (k point grid)]]
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### 2.4 SCF algorithms, tuning and stopping criteria
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1. [[topic:SCFAlgorithms|SCF algorithms]]
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2. [[topic:SCFControl|SCF control, tolerances and stopping criteria]]
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3. [[topic:ForcesStresses|Forces and stresses]]
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4. [[topic:TuningSpeedMem|Tuning speed and memory usage]]
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5. [[topic:Recursion|Recursion methods and orbital free calculations]] (not in production)
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### 2.5 Added electric/magnetic field, other artificial constraints/modifications, and related properties ...
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1. [[topic:Berry|Electric polarization and finite electric field]]
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2. [[topic:MagField|External magnetic field]]
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3. [[topic:ConstrainedDFT|Constrained Density-Functional Theory]]
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4. [[topic:MagMom|Constrained atomic magnetic moment]]
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5. [[topic:EFG|Electric field gradients and Mossbauer Fermi contact interaction]]
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6. [[topic:Artificial|Artificial modifications of the system]]
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## 3 Global control parameters: flow, parallelism, output files, output content, timing and memory control ...
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1. [[topic:multidtset|Multi-dataset calculations]]
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2. [[topic:parallelism|Parallelism and ABINIT]]
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3. [[topic:printing|Printing files]]
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4. [[topic:Output|Tuning the output content in different files]]
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5. [[topic:Control|Time and memory control]]
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## 4 Molecular dynamics, geometry optimization, transition paths
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1. [[topic:GeoOpt|Geometry optimization]]
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2. [[topic:MolecularDynamics|Molecular dynamics]]
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3. [[topic:TransPath|Transition path searches: NEB and string method]]
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4. [[topic:GeoConstraints|Geometry constraints]]
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5. [[topic:PIMD|Path-integral molecular dynamics (PIMD)]]
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5. [[topic:CrossingBarriers|Crossing barrier search, linear combination of constrained DFT energies and ensemble DFT]]
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6. [[topic:LOTF|Learn-on-the-flight (LOTF)]] (not in production)
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## 5 Correlated electrons
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When correlated electrons are to be considered (in most cases, when *d* and *f*
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orbitals play an active role), it is necessary to go beyond the standard DFT
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framework. ABINIT enables the following possibilities:
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1. [[topic:Hybrids|Hybrid functionals]]
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2. [[topic:DFT+U|DFT+U approximation]]
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3. [[topic:DMFT|Dynamical Mean Field Theory (DMFT)]]
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4. [[topic:CalcUJ|Calculation of the Hubbard U and Hund J]]
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## 6 Adiabatic response properties (phonons, low-frequency dielectric, Raman, elasticity, temperature dependence ...)
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Many properties can be obtained in the approximation that the electrons **stay
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in their ground state** (adiabatic responses). The poweful Density-Functional
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Perturbation Theory (DFPT) framework allows ABINIT to address directly all
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such properties in the case that are connected to derivatives of the total
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energy with respect to some perturbation. This includes all dynamical effects
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due to phonons and their coupling, thus also temperature-dependent properties due to phonons.
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1. [[topic:DFPT|Generalities about DFPT]]
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2. [[topic:q-points|Wavevectors for phonons (q-points)]]
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3. [[topic:Phonons|Vibrational and dielectric properties
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(phonon frequencies and modes, IR and Raman spectra, Born effective charges)]]
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4. [[topic:PhononBands|Phonon bands and DOS, interatomic force constants, sound velocity]]
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5. [[topic:Temperature|Temperature dependent properties (free energy, entropy, specific heat,
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atomic temperature factors, thermal expansion)]]
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6. [[topic:Elastic|Elasticity and piezoelectricity]]
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7. [[topic:longwave|Long wave method: quadrupoles, flexoelectricity, optical activity]]
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8. [[topic:nonlinear|Raman intensities and electro-optic properties]]
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9. [[topic:ElPhonInt|Electron-phonon interaction]]
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10. [[topic:PhononWidth|Phonon linewidth due to the electron-phonon interaction]]
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11. [[topic:ElPhonTransport|Electronic transport properties from electron-phonon interaction
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(resistivity, superconductivity, thermal)]]
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12. [[topic:TDepES|Temperature dependence of the electronic structure from electron-phonon interaction]]
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13. [[topic:ConstrainedPol|Constrained polarization geometry optimization]] (advanced topic)
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## 7 Excited state calculations, and frequency-dependent electronic and optical properties
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Excited-state calculations and frequency-dependent properties (for frequencies
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that are non-negligible with respect to the electronic gap), can be addressed
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by a variety of methodologies, usually trading accuracy for speed. At the
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lowest level, one encounters the independent-particle approximation, building
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upon some previously obtained band structure (e.g. Kohn-Sham band structure
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from DFT). For charged excitations, allowing to obtain a quasiparticle band
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structure without the well-known DFT band gap problem, one has to resort to
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(costly) GW calculations. For neutral excitations (i.e. optical), the (costly)
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Bethe-Salpeter approach is the most accurate formalism presently available.
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TDDFT and Δ-SCF calculations are cheaper but will work well for molecules and
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for isolated defects in a solid, not for e.g. correcting the band gap.
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1. [[topic:Optic|Linear and non-linear optical properties in the independent-particle approximation]]
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2. [[topic:FrequencyMeshMBPT|Definition of frequency meshes for Many-body perturbation theory]]
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3. [[topic:Susceptibility|Frequency-dependent susceptibility matrix, and related dielectric matrix and screened Coulomb interaction]]
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4. [[topic:SelfEnergy|Electronic self-energy]]
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5. [[topic:GW|GW calculations for accurate band structure, including self-consistency]]
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6. [[topic:BSE|Bethe-Salpeter calculations for accurate optical properties]]
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7. [[topic:TDDFT|TDDFT calculations]]
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8. [[topic:DeltaSCF|DeltaSCF calculations]]
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9. [[topic:RandStopPow|Random electronic stopping power]]
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10. [[topic:GWls|GW- Lanczos-Sternheimer method]] (not in production)
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## 8 Second-principles calculations with MULTIBINIT: handling millions of atoms with first-principles accuracy
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By constructing model Hamiltonians whose linear and selected non-linear characteristics
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are identical to those from first-principles calculations, and simulating millions
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of atoms with these model Hamiltonians, one can study phase transitions, polarization boundaries,
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and other properties for large-scale systems that cannot be reached from first-principles algorithms
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implemented in ABINIT and most DFT codes. Even with respect to linear-scaling codes, the prefactor
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is much smaller.
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This is implemented in the MULTIBINIT application.
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1. [[topic:LatticeModel|Lattice model at the harmonic level]]
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2. [[topic:FitProcess|FitProcess]]
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3. [[topic:BoundingProcess|BoundingProcess]]
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4. [[topic:DynamicsMultibinit|DynamicsMultibinit]]
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## 9 Electronic properties and analysis tools (DOS, STM, Wannier, band plotting and interpolating...)
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Many properties are directly deduced from the knowledge of the electronic
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wavefunctions, eigenenergies, density, potential, etc. Some necessitates
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additional tuning parameters or are linked to postprocessing tools and are
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described in the following topics. Some others are actually activated through
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a single printing parameter, such as the Electron Localization Function (ELF -
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see [[prtelf]]). See the list of "printing" input variables in [[topic:printing]].
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1. [[topic:ElecBandStructure|Electronic band structure and related topics]]
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2. [[topic:ElecDOS|Electronic DOS and related topics]]
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3. [[topic:EffectiveMass|Effective mass calculations]]
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4. [[topic:Unfolding|Unfolding supercell band structures]]
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5. [[topic:DensityPotential|Manipulating the density and potential]]
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6. [[topic:Macroave|Macroscopic average of density and potential]]
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7. [[topic:STM|Scanning Tunneling Microscopy map]]
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8. [[topic:Wannier|Wannier functions]]
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9. [[topic:Bader|Bader Atom-In-Molecule analysis]]
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9. [[topic:AtomCentered|Atom centered properties - Charge, Magnetization, etc]]
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## 10 Other physical properties (e.g. positron)
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1. [[topic:positron|Positron calculations]]
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2. [[topic:LDAminushalf|The LDA-1/2 approach]]
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## 11 Analysis/postprocessing tools
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1. [[topic:Abipy|Abipy - ABINIT swiss knife]]
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2. [[topic:APPA|Abinit Post-Processor Application (APPA), for molecular-dynamics trajectory analysis]]
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3. [[topic:Band2eps|Band2eps for phonon dispersion curves]]
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4. [[topic:aTDEP|Temperature Dependent Effective Potential, for thermodynamical properties]]
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## 12 Miscellaneous topics
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1. [[topic:Verification|Verification of the implementation]]
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2. [[topic:PortabilityNonRegression|Portability and non-regression tests]]
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3. [[topic:Git|Git, gitlab and github for the ABINIT project]]
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4. [[topic:Dev|Miscellaneous for developers]]
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