# ABINIT, PAW input variables:

## List and description.

This document lists and provides the description of the name (keywords) of the input variables for runs based on the Projector Augmented Waves methodology, 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) 1998-2006 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 .

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

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
- Response Function variables, VARRF
- Structure optimization variables, VARRLX
- Wannier fonctions variables, VARWAN

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

A.
B. bxctmindg
C.
D.
E.
F.
G.
H.
I. iboxcut
J.
K.
L.
M. mqgriddg
N. ngfftdg
O.
P. pawecutdg pawlcutd pawlmix pawmixdg pawnphi pawntheta pawnzlm pawoptmix pawovlp pawprtvol pawsphmix pawxcdev
Q.
R.
S.
T.
U.
V.
W.
X.
Y.
Z.

bxctmindg
Mnemonics: BoX CuT-off MINimum for the Double Grid (PAW)
Characteristic:
Variable type: real parameter
Default is 2.0
Needed only when usepaw=1.
The box cut-off ratio is the ratio between the wavefunction plane wave sphere radius, and the radius of the sphere that can be inserted in the FFT box, in reciprocal space.
If the density were only generated from wavefunctions, in order for the density to be exact, this ratio should be near two. If one uses a smaller ratio, one will gain speed, at the expense of accuracy. In case of pure ground state calculation (e.g. for the determination of geometries), this is sensible. However, the wavefunctions that are obtained CANNOT be used for starting response function calculation.
However, some augmentation charge is always added in PAW, and even with the box cut-off ratio larger than two, the density is never exact. Sometimes, this ratio must be much larger than two for the computation to be converged at the required level of accuracy.
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iboxcut
Mnemonics:
Characteristic:
Variable type: integer parameter
Default is 0
Concern all summations in the reciprocal space and is allowed in PAW and norm-conserving.

- if set to 0 all reciprocal space summations are done in a sphere contained in the FFT box.
- if set to 1 all reciprocal space summations are done in the whole FFT box (usefull for tests).

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mqgriddg
Mnemonics:
Characteristic:
Variable type: integer parameter
Default is 3001
Maximum number of wavevectors used to sample the local part of the potential, in PAW. Actually referred to as mqgrid_vl internally. Should change name to the latter ...
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ngfftdg
Mnemonics: Number of Grid points for Fast Fourier Transform : Double Grid
Characteristic:
Variable type: integer array ngfftdg(3)
Default is 0 0 0 (so, automatic selection of optimal values)
Needed only when usepaw=1.
This variable has the same meaning as ngfft (gives the size of fast Fourier transform (fft) grid in three dimensions) but concerns the "double grid" only used for PAW calculations.
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pawecutdg
Mnemonics: PAW - Energy CUToff for the Double Grid
Characteristic: ENERGY
Variable type: real parameter
Default is 2 times ecut
Needed only when usepaw=1.
Define the energy cut-off for the fine FFT grid (the "double grid", that allows to transfer data from the normal, coarse, FFT grid to the spherical grid around each atom).
pawecutdg must be larger or equal to ecut. If it is equal to it, then no fine grid is used. The results are not very accurate, but the computations proceed quite fast.
The default value is sometimes a bit too low, but does not slow down the computation. The choice made for this variable DOES have a bearing on the numerical accuracy of the results, and, as such, should be the object of a convergence study. The convergence test might be made on the total energy or derived quantities, like forces, but also on the two values of the "Compensation charge inside spheres", a quantity written in the log file.
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pawlcutd
Mnemonics: PAW - 1+L angular momentum used to CUT the development in moments of the Densitites
Characteristic:
Variable type: integer parameter
Default is 10
Needed only when usepaw=1.
The expansion of the densities in angular momenta is performed up to l=pawlcutd-1.
The choice made for this variable DOES have a bearing on the numerical accuracy of the results, and, as such, should be the object of a convergence study. The convergence test might be made on the total energy or derived quantities, like forces, but also on the two values of the "Compensation charge inside spheres", a quantity written in the log file.
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pawlmix
Mnemonics: PAW - maximum L used in the spherical part MIXing
Characteristic:
Variable type: integer parameter
Default is 10
Needed only when usepaw=1.
The choice made for this variable determine how the spherical part of the density is mixed during electronic iterations.
Only parts of augmentation occupancies (rhoij quantities) associated with l angular momenta up to l=pawlmix are mixed. Other parts of augmentation occupancies are not included in the mixing process.
This option is usefull to save CPU time but DOES have a bearing on the numerical accuracy of the results.
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pawmixdg
Mnemonics: PAW - MIXing is done (or not) on the (fine) Double Grid
Characteristic:
Variable type: integer parameter
Default is 1
Needed only when usepaw=1.
The choice made for this variable determine on which grid the density (or potential) is mixed during the SCF cycle.
- If pawmixdg=1 the density/potential is mixed in REAL space using the fine FFT grid (defined by pawecutdg or ngfftdg).
- If pawmixdg=0 the density/potential is mixed in RECIPROCAL space using the coarse FFT grid (defined by ecut or ngfft). Only components of the coarse grid are mixed using the scheme defined by iscf; other components are only precondionned by diemix and simply mixed.
This option is usefull to save memory and does not affect numerical accuracy of converged results. If pawmixdg=1, density and corresponding residual are stored for previous iterations and are REAL arrays of size %nfftdg. If pawmixdg=0, density and corresponding residual are stored for previous iterations and are COMPLEX arrays of size %nfft. The memory saving is particulary efficient when using the Pulay mixing (iscf=7 or 17).
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pawnphi
Mnemonics: PAW - Number of PHI angles used to discretize the sphere around each atom.
Characteristic:
Variable type: integer parameter
Default is 13
Needed only when usepaw=1.
Number of phi angles (longitude) used to discretize the data on the atomic spheres. This discretization is completely defined by pawnphi and pawntheta.
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pawntheta
Mnemonics: PAW - Number of THETA angles used to discretize the sphere around each atom.
Characteristic:
Variable type: integer parameter
Default is 12
Needed only when usepaw=1.
Number of theta angles (latitude) used to discretize the data on the atomic spheres. This discretization is completely defined by pawntheta and pawnphi.
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pawnzlm
Mnemonics: PAW - only compute Non-Zero LM-moments of spherical density
Characteristic:
Variable type: integer parameter
Default is 1
Needed only when usepaw=1.
Concerns the computation of the spherical density (named rho_1 - rho_tild_1).
If set to 0, all lm-moments of the spherical density are computed at each electronic iteration br> If set to 1, only non-zero lm-moments of the spherical density are computed at each electronic iteration (they are all computed at first iteration then only the non-zero ; thus the first iteration is more cpu expensive)
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pawoptmix
Mnemonics: PAW - OPTion for the MIXing of the spherical part
Characteristic:
Variable type: integer parameter
Default is 0
Needed only when usepaw=1.
When PAW is activated, the self-consistent requires the mixing of both the total potential (or density) and the "spherical part" (in fact the augmentation occupancies rho_ij).
The same mixing scheme is applied to the potential (density) and the spherical part. It is optimized in order to minimize a residual.
If pawoptmix=0 the residual is the potential (or density) residual.
If pawoptmix=1 the residual is a sum of the potential (or density) residual and the "spherical part" residual.
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pawovlp
Mnemonics: PAW - spheres OVerLap allowed (in percentage)
Characteristic:
Variable type: real parameter
Default is 5.0
Needed only when usepaw=1.
When PAW is activated, a localized atomic basis is added to describe wave functions. Spheres around atoms are defined and they are IN THEORY not allowed to overlap. However, a little overlap can be allowed without compromising accurary of results. Be aware that to high overlaps can lead to unphysical results.
With the pawovlp variable, the user can control the (voluminal) overlap percentage allowed without stopping the execution.
pawovlp is the value (in percentage: 0...100%) obtained by dividing the volume of the overlap of two spheres by the volume of the smallest sphere.
The following values are permitted for pawovlp:
- pawovlp<0. : overlap is always allowed
- pawovlp=0. : no overlap is allowed
- pawovlp>0. and <100. : overlap is allowed only if it is less than pawovlp %
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pawprtvol
Mnemonics: PAW PRinT VOLume
Characteristic:
Variable type: integer parameter
Default is
Needed only when usepaw=1.
Control print volume and debugging output for PAW. If set to 0, the print volume is at its minimum. pawprtvol can have values from 0 to 3.
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pawsphmix
Mnemonics: PAW - SPHerical part MIXing factor
Characteristic:
Variable type: real parameter
Default is equal to diemix
Needed only when usepaw=1.
pawsphmix is the factor used when preconditionning the spherical part residual (the rhoij residual, where rhoij are the augmentation occupancies)
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pawxcdev
Mnemonics: PAW - choice for eXchange-Correlation DEVelopment (spherical part)
Characteristic:
Variable type: integer parameter
Default is 1
Needed only when usepaw=1.

- If set to 0, the exchange-correlation term in the spherical part of energy is totally computed on the angular mesh
- If set to 1, the exchange-correlation term in the spherical part of energy is developped onto lm-moments (at order 1)
- If set to 2, the exchange-correlation term in the spherical part of energy is developped onto lm-moments (at order 2) - not yet available

Be careful: GGA requires pawxcdev > 0
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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