# ABINIT, response function input variables:

## List and description.

This document lists and provides the description of the name (keywords) of the response function input variables to be used in the main input file of the abinis code.
The new user is advised to read first the new user's guide, before reading the present file. It will be easier to discover the present file with the help of the tutorial.
When the user is sufficiently familiarized with ABINIT, the reading of the ~abinit/doc/users/tuning file might be useful. For response-function calculations using abinis, please read the response function help file

##### Copyright (C) 1998-2009 ABINIT group (DCA, XG, RC)
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.txt .

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Help files : New user's guide | Abinis (main) | Abinis (respfn) | Mrgddb | Anaddb | AIM (Bader) | Cut3D | Optic | Mrgscr

Files that describe other input variables:

- Basic variables, VARBAS
- Developper variables, VARDEV
- File handling variables, VARFIL
- Geometry builder + symmetry related variables, VARGEO
- Ground-state calculation variables, VARGS
- GW variables, VARGW
- Internal variables, VARINT
- Parallelisation variables, VARPAR
- Projector-Augmented Wave variables, VARPAW
- Structural optimization variables, VARRLX
- Wannier90 interface variables, VARW90

### Content of the file : alphabetical list of variables.

A.
B.
C.
D. dsifkpt
E.
F. frzfermi
G.
H.
I. ieig2rf
J.
K.
M. mkqmem mk1mem
N.
O.
P. prepanl prtbbb
Q.
R. rfasr rfatpol rfdir rfelfd rfmeth rfphon rfstrs rfthrd rfuser rf1atpol rf1dir rf1elfd rf1phon rf2atpol rf2dir rf2elfd rf2phon rf3atpol rf3dir rf3elfd rf3phon
S. sciss
T. td_maxene td_mexcit
U.
V.
W.
X.
Y.
Z.

dsifkpt
Mnemonics: DenSiFy K-PoinTs
Characteristic:
Variable type: integer array dsifkpt(3)
Default is 1.
NOT USED ANYMORE

Can be used to density the k point grid along the lines that are parallel to the three primitive vectors, in reciprocal space. Should be useful for third-order derivatives that include some derivative with respect to k-points or electric field. This part is in development.

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frzfermi
Mnemonics: FReeZe FERMI energy
Characteristic:
Variable type: integer parameter
Default is 0.
Can be used to suppress artificially the first-order change of Fermi energy, in case of Response Function calculation for metals at Q=0. This change is needed, but was not computed prior to v4.4 . Its calculation has been implemented by DHamann. The input variable frzfermi, if set to 1, allows to recover the previous, incorrect behaviour.
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ieig2rf
Mnemonics: Integer for second-order EIGenvalues from Response-Function
Characteristic: RESPFN
Variable type: integer parameters
Default is 0.
TO BE DESCRIBED
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mkqmem
Mnemonics: Maximum number of K+Q - points in MEMory

mk1mem
Mnemonics: Maximum number of K - points for 1st order wavefunctions, kept in MEMory
Characteristic: RESPFN
Variable type: integer parameters
Default is nkpt, i.e. in-core solution.
Plays a role similar to mkmem but for different sets of wavefunctions : the ground state wavefunctions at k+q and the first-order wavefunctions. Only needed for response calculations.
Internal representation as mkmems(2) and mkmems(3).
Note (991019) that although the effective number of k points can be reduced thanks to symmetry for different perturbations, mkqmem and mk1mem are presently still compared with the input nkpt. This should be changed later.
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prepanl
Mnemonics: PREPAre Non-Linear response calculation
Characteristic: RESPFN
Variable type: integer parameter
Default is 0.
The computation of third-order derivatives from the 2n+1 theorem requires the first-order wavefunctions and densities obtained from a linear response calculation. The standard approach in a linear response calculation is (i) to compute only the irreducible perturbations, and (ii) to use symmetries to reduce the number of k-points for the k-point integration.
This approach cannot be applied, presently (v4.1), if the first-order wavefunctions are to be used to compute third-order derivatives. First, for electric fields, the code needs the derivatives along the three directions. Still, in case of phonons, only the irreducible perturbations are required. Second, for both electric fields and phonons, the wavefunctions must be available in half the BZ (kptopt=2), or the full BZ (kptopt=3).
During the linear response calculation, in order to prepare a non-linear calculation, one should put prepanl to 1 in order to force ABINIT (i) to compute the electric field perturbation along the three directions explicitly, and (ii) to keep the full number of k-points.
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prtbbb
Mnemonics: PRinT Band-By-Band decomposition
Characteristic: RESPFN
Variable type: integer parameter
Default is 0.
If prtbbb is 1, print the band-by-band decomposition of Born effective charges and localization tensor, in case they are computed. See Ph. Ghosez and X. Gonze, J. Phys.: Condens. Matter 12, 9179 (2000).
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rfasr
Mnemonics: Response Function : Acoustic Sum Rule
Characteristic: RESPFN
Variable type: integer parameter
Default is 0.
Control the evaluation of the acoustic sum rule in effective charge calculations within a response function calculation.

- 0 => no acoustic sum rule imposed
- 1 => acoustic sum rule imposed with extra charge evenly distributed among atoms
- 2 => acoustic sum rule imposed with extra charge given proportionally to those atoms with the largest effective charge.

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rfatpol
Mnemonics: Response Function : limits of ATomic POLarisations
Characteristic: RESPFN

rf1atpol
Mnemonics: non-linear Response Function, 1st mixed perturbation : limits of ATomic POLarisations
Characteristic: NON-LINEAR

rf2atpol
Mnemonics: non-linear Response Function, 2nd mixed perturbation : limits of ATomic POLarisations
Characteristic: NON-LINEAR

rf3atpol
Mnemonics: non-linear Response Function, 3rd mixed perturbation : limits of ATomic POLarisations
Characteristic: NON-LINEAR

Variable type: integer array of 2 elements
Default is 1 1
Control the range of atoms for which displacements will be considered in phonon calculations (atomic polarisations), or in non-linear computations, using the 2n+1 theorem.
These values are only relevant to phonon response function calculations, or non-linear computations.
May take values from 1 to natom, with rfatpol(1)<=rfatpol(2).
The atoms to be moved will be defined by the
do-loop variable iatpol :
do iatpol=rfatpol(1),rfatpol(2)
For the calculation of a full dynamical matrix, use rfatpol(1)=1 and rfatpol(2)=natom, together with rfdir 1 1 1 . For selected elements of the dynamical matrix, use different values of rfatpol and/or rfdir. The name 'iatpol' is used for the part of the internal variable ipert when it runs from 1 to natom. The internal variable ipert can also assume values larger than natom, of electric field or stress type (see respfn.help).
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rfdir
Mnemonics: Response Function : DIRections
Characteristic: RESPFN

rf1dir
Mnemonics: non-linear Response Function, 1st mixed perturbation : DIRections
Characteristic: NON-LINEAR

rf2dir
Mnemonics: non-linear Response Function, 2nd mixed perturbation : DIRections
Characteristic: NON-LINEAR

rf3dir
Mnemonics: non-linear Response Function, 3rd mixed perturbation : DIRections
Characteristic: NON-LINEAR

Variable type: integer array of 3 elements
Default is 0 0 0.
Gives the directions to be considered for response function calculations, or non-linear computations (also for the Berry phase computation of the polarization, see the berryopt input variable).
The three elements corresponds to the three primitive vectors, either in real space (phonon calculations), or in reciprocal space (d/dk and homogeneous electric field calculations). So, they generate a basis for the generation of the dynamical matrix or to macroscopic dielectric tensor, of the effective charge tensors.
If equal to 1, response functions, as defined by rfelfd, rfphon, rfdir and rfatpol, are to be computed for the corresponding direction. If 0, this direction should not be considered (for non-linear computations, the corresponding input variables should be used).
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rfelfd
Mnemonics: Response Function with respect to the ELectric FielD
Characteristic: RESPFN

rf1elfd
Mnemonics: non-linear Response Function, 1st mixed perturbation : ELectric FielD
Characteristic: NON-LINEAR

rf2elfd
Mnemonics: non-linear Response Function, 2nd mixed perturbation : ELectric FielD
Characteristic: NON-LINEAR

rf3elfd
Mnemonics: non-linear Response Function, 3rd mixed perturbation : ELectric FielD
Characteristic: NON-LINEAR

Variable type: integer parameter
Default is 0.
Turns on electric field response function calculations (or non-linear computation, including the electric field perturbation). Actually, such calculations requires first the non-self-consistent calculation of derivatives with respect to k, independently of the electric field perturbation itself.

- 0=>no electric field perturbation
- 1=>full calculation, with first the derivative of ground-state wavefunction with respect to k (d/dk calculation), by a non-self-consistent calculation, then the generation of the first-order response to an homogeneous electric field
- 2=>only the derivative of ground-state wavefunctions with respect to k
- 3=>only the generation of the first-order response to the electric field, assuming that the data on derivative of ground-state wavefunction with respect to k is available on disk.

(Note : because the tolerances to be used for derivatives or homogeneous electric field are different, one often does the calculation of derivatives in a separate dataset, followed by calculation of electric field response as well as phonon.
The options 2 and 3 proves useful in that context ; also, in case a scissor shift is to be used, it is usually not applied for the d/dk response).
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rfmeth
Mnemonics: Response Function METHod
Characteristic: RESPFN
Variable type: integer parameter
Default is 1.
Selects method used in response function calculations. Presently, only 1 is allowed.
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rfphon
Mnemonics: Response Function with respect to PHONons
Characteristic: RESPFN

rf1phon
Mnemonics: non-linear Response Function, 1st mixed perturbation : PHONons
Characteristic: NON-LINEAR

rf2phon
Mnemonics: non-linear Response Function, 2nd mixed perturbation : PHONons
Characteristic: NON-LINEAR

rf3phon
Mnemonics: non-linear Response Function, 3rd mixed perturbation : PHONons
Characteristic: NON-LINEAR

Variable type: integer parameter
Default is 0.
It must be equal to 1 to run phonon response function calculations, or to include some phonon perturbation in non-linear computations.
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rfstrs
Mnemonics: Response Function with respect to STRainS
Characteristic: RESPFN
Variable type: integer parameter
Default is 0.
Used to run strain response-function calculations (e.g. needed to get elastic constants). Define, with rfdir, the set of perturbations.

- 0=>no strain perturbation
- 1=>only uniaxial strain(s) (ipert=natom+3 is activated)
- 2=>only shear strain(s) (ipert=natom+4 is activated)
- 3=>both uniaxial and shear strain(s) (both ipert=natom+3 and ipert=natom+4 are activated)

See the possible restrictions on the use of strain perturbations, in the respfn_help file.
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rfthrd
Mnemonics: Response Function of THiRD order
Characteristic: RESPFN
Variable type: integer parameter
Default is 0.
Used to control response function calculation of third order response.
Not implemented.
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rfuser
Mnemonics: Response Function, USER-defined
Characteristic: RESPFN
Variable type: integer parameter
Default is 0.
Available to the developpers, to activate the use of ipert=natom+5 and ipert=natom+6, two sets of perturbations that the developpers can define.

- 0=>no computations for ipert=natom+5 or ipert=natom+6
- 1=>response with respect to perturbation natom+5 will be computed
- 2=>response with respect to perturbation natom+6 will be computed
- 3=>responses with respect to perturbations natom+5 and natom+6 will be computed

In order to define and use correctly the new perturbations, the developper might have to include code lines or additional routines at the level of the following routines : cgwf3.f, chkph3.f, dyout3.f, d2sym3.f, eneou3.f, eneres3.f, gath3.f, insy3.f, loper3.f, mkcor3.f, nstdy3.f, nstwf3.f, respfn.f, scfcv3.f, syper3.f, vloca3.f, vtorho3.f, vtowfk3.f, wings3.f, . In these routines, the developper should pay a particular attention to the rfpert array, defined in the routine respfn.f , as well as to the ipert local variable.

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sciss
Mnemonics: SCISSor operator
Characteristic: RESPFN, ENERGY
Variable type: real parameter
Default is 0.
It is the value of the "scissors operator", the shift of conduction band eigenvalues, used in response function calculations.
Can be specified in Ha (the default), Ry, eV or Kelvin, since ecut has the 'ENERGY' characteristics. (1 Ha=27.2113845 eV)
Typical use is for response to electric field (rfelfd=3), but NOT for d/dk (rfelfd=2) and phonon responses.
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td_maxene
Mnemonics: Time-Dependent dft : MAXimal kohn-sham ENErgy difference
Characteristic: TDDFT
Variable type: real parameter
Default is huge.
The Matrix to be diagonalized in the Casida framework (see "Time-Dependent Density Functional Response Theory of Molecular systems: Theory, Computational Methods, and Functionals", by M.E. Casida, in Recent Developments and Applications of Modern Density Functional Theory, edited by J.M. Seminario (Elsevier, Amsterdam, 1996).) is a NxN matrix, where, by default, N is the product of the number of occupied states by the number of unoccupied states.
The input variable td_maxene allows to diminish N : it selects only the pairs of occupied and unoccupied states for which the Kohn-Sham energy difference is less than td_maxene.
See td_mexcit for an alternative way to decrease N.
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td_mexcit
Mnemonics: Time-Dependent dft : Maximal number of EXCITations
Characteristic: TDDFT
Variable type: real parameter
Default is 0.
The Matrix to be diagonalized in the Casida framework (see "Time-Dependent Density Functional Response Theory of Molecular systems: Theory, Computational Methods, and Functionals", by M.E. Casida, in Recent Developments and Applications of Modern Density Functional Theory, edited by J.M. Seminario (Elsevier, Amsterdam, 1996).) is a NxN matrix, where, by default, N is the product of the number of occupied states by the number of unoccupied states.
The input variable td_mexcit allows to diminish N : it selects the first td_mexcit pairs of occupied and unoccupied states, ordered with respect to increasing Kohn-Sham energy difference. However, when td_mexcit is zero, all pairs are allowed.
See td_maxene for an alternative way to decrease N.
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Goto : ABINIT home Page | Suggested acknowledgments | List of input variables | Tutorial home page | Bibliography

Help files : New user's guide | Abinis (main) | Abinis (respfn) | Mrgddb | Anaddb | AIM (Bader) | Cut3D | Optic | Mrgscr