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Subsections

5.1 Electronic structure calculations

5.1.0.1 Single-point (fixed-ion) SCF calculation

Set calculation='scf' (this is actually the default). Namelists &IONS and &CELL will be ignored. See Example 01.

5.1.0.2 Band structure calculation

First perform a SCF calculation as above; then do a non-SCF calculation with the desired k-point grid and number nbnd of bands. Use calculation='bands' if you are interested in calculating only the Kohn-Sham states for the given set of k-points (e.g. along symmetry lines: see for instance http://www.cryst.ehu.es/cryst/get_kvec.html). Specify instead calculation='nscf' if you are interested in further processing of the results of non-SCF calculations (for instance, in DOS calculations). In the latter case, you should specify a uniform grid of points. For DOS calculations you should choose occupations='tetrahedra', together with an automatically generated uniform k-point grid (card K_POINTS with option ``automatic''). Specify nosym=.true. to avoid generation of additional k-points in low symmetry cases. Variables prefix and outdir, which determine the names of input or output files, should be the same in the two runs. See Examples 01, 05, 08,

NOTA BENE: until v.4.0, atomic positions for a non scf calculations were read from input, while the scf potential was read from the data file of the scf calculation. Since v.4.1, both atomic positions and the scf potential are read from the data file so that consistency is guaranteed.

5.1.0.3 Noncolinear magnetization, spin-orbit interactions

The following input variables are relevant for noncolinear and spin-orbit calculations:

noncolin
lspinorb
starting_magnetization (one for each type of atoms)
To make a spin-orbit calculation noncolin must be true. If starting_magnetization is set to zero (or not given) the code makes a spin-orbit calculation without spin magnetization (it assumes that time reversal symmetry holds and it does not calculate the magnetization). The states are still two-component spinors but the total magnetization is zero.

If starting_magnetization is different from zero, it makes a non collinear spin polarized calculation with spin-orbit interaction. The final spin magnetization might be zero or different from zero depending on the system.

Furthermore to make a spin-orbit calculation you must use fully relativistic pseudopotentials at least for the atoms in which you think that spin-orbit interaction is large. If all the pseudopotentials are scalar relativistic the calculation becomes equivalent to a noncolinear calculation without spin orbit. (Andrea Dal Corso, 2007-07-27) See Example 13 for non-collinear magnetism, Example 22 for spin-orbit interactions.

5.1.0.4 DFT+U

DFT+U (formerly known as LDA+U) calculation can be performed within a simplified rotationally invariant form of the U Hubbard correction. See Example 25 and references quoted therein.

5.1.0.5 Dispersion Interactions (DFT-D)

For DFT-D (DFT + semiempirical dispersion interactions), see the description of input variables london*, sample files tests/vdw.*, and the comments in source file Modules/mm_dispersion.f90.

5.1.0.6 Hartree-Fock and Hybrid functionals

Calculations in the Hartree-Fock approximation, or using hybrid XC functionals that include some Hartree-Fock exchange, currently require that -DEXX is added to the preprocessing options DFLAGS in file make.sys before compilation (if you change this after the first compilation, make clean, recompile). Documentation on usage can be found in subdirectory examples/EXX_example/.

The algorithm is quite standard: see for instance Chawla and Voth, JCP bf 108, 4697 (1998); Sorouri, Foulkes and Hine, JCP 124, 064105 (2006); Spencer and Alavi, PRB 77, 193110 (2008). Basically, one generates auxiliary densities $ \rho_{{-q}}^{}$ = $ \phi^{{*}}_{{k+q}}$*$ \psi_{k}^{}$ in real space and transforms them to reciprocal space using FFT; the Poisson equation is solved and the resulting potential is transformed back to real space using FFT, then multiplied by $ \phi_{{k+q}}^{}$ and the results are accumulated. The only tricky point is the treatment of the q $ \rightarrow$ 0 limit, which is described in the Appendix A.5 of the QUANTUM ESPRESSO paper mentioned in the Introduction (note the reference to the Gygi and Baldereschi paper). See also J. Comp. Chem. 29, 2098 (2008); JACS 129, 10402 (2007) for examples of applications.

5.1.0.7 Dispersion interaction with non-local functional (vdwDF)

See example examples/example_vwdDF/ and references quoted in file README therein.

5.1.0.8 Polarization via Berry Phase

See Example 10, file example10/README, and the documentation in the header of PW/bp_c_phase.f90.

5.1.0.9 Finite electric fields

There are two different implementations of macroscopic electric fields in pw.x: via an external sawtooth potential (input variable tefield=.true.) and via the modern theory of polarizability (lelfield=.true.). The former is useful for surfaces, especially in conjunction with dipolar corrections (dipfield=.true.): see examples/dipole_example for an example of application. Electric fields via modern theory of polarization are documented in example 31. The exact meaning of the related variables, for both cases, is explained in the general input documentation.


next up previous contents
Next: 5.2 Optimization and dynamics Up: 5 Using PWscf Previous: 5 Using PWscf   Contents
buildd user 2011-12-13