Exact exchange¶
Currently we have two implementations of exact exchange:
hybrid.py: Can handle Gammapoint only calculations selfconsistently (for molecules and large cells).
exx.py: Can handle kpoints, but not selfconsitently.
Contents
Selfconsistent finitedifference implementation¶
The current implementation lacks the following features:
Support for periodic systems. Actually, the code won’t complain, but the results have not been tested.
Support for kpoint sampling. No consideration has been made as to multiple kpoints, or even comlplex wave functions, so this definitely won’t work.
Forces. Force evaluations when including (a fraction of ) the fock operator in the xcfunctional has been implemented, but has not been tested.
Speed. Inclusion of Fock exchange is exceedingly slow. The bottleneck is solving the poisson integrals of the Fock operator, which is currently done using an iterative realspace solver with a zero initial guess for the potential at each SCF cycle. This should be optimized.
One way to speed up an exactexchange (or hybrid) calculation is to use the
coarse grid (used for wave functions) instead of the finegrid (used for for
densities) for the Fock potentials. This should give a speedup factor of 8.
This can be specified in the xc
keyword like in this example
coarse.py
Parallelization using domain decomposition is fully supported.
The Fock operator can be used to do the hybrid functional PBE0, and of course,
HartreeFock type EXX. These are accessed by setting the xc
keyword to
PBE0
or EXX
respectively.
The following functionals are suppported:
Functional 
Type 
Reference 

EXX 
Global 

PBE0 
Global 

B3LYP 
Global 

HSE03 
RSFSR 

HSE06 
RSFSR 

CAMYB3LYP 
RSFLR 

CAMYBLYP 
RSFLR 

CAMYB3LYP 
RSFLR 

LCYBLYP 
RSFLR 

LCYPBE 
RSFLR 
Here “Global” denotes global hybrids, which use the same percentage of HartreeFock exchange for every point in space, while “RSFSR” and “RSFLR” denotes rangeseparated functionals which mix the fraction of HartreeFock and DFT exchange based on the spatial distance between two points, where for a “RSFSR” the amount of HartreeFock exchange decrease with the distance and increase for a “RSFLR”. See Range separated functionals (RSF) for more detailed information on RSF(LR).
A thesis on the implementation of EXX in the PAW framework, and the specifics of the GPAW project can be seen on the literature page.
A comparison of the atomization energies of the g21 testset calculated in VASP, Gaussian03, and GPAW is shown in the below two figures for the PBE and the PBE0 functional respectively.
In the last figure, the curve marked GPAW (nonself.)
is a non
selfconsistent PBE0 calculation using selfconsistent PBE orbitals.
It should be noted, that the implementation lacks an optimized effective
potential. Therefore the unoccupied states utilizing EXX as implemented in
GPAW usually approximate (excited) electron affinities. Therefore calculations
utilizing HartreeFock exchange are usually a bad basis for the calculation of
optical excitations by lrTDDFT. As a remedy, the improved virtual orbitals
(IVOs, [HA71]) were implemented. The requested excitation basis can be chosen
by the keyword excitation
and the state by excited
where the state is
counted from the HOMO downwards:
"""Calculate the excitation energy of NaCl by an RSF using IVOs."""
from ase.build import molecule
from ase.units import Hartree
from gpaw import GPAW, setup_paths
from gpaw.mpi import world
from gpaw.occupations import FermiDirac
from gpaw.test import equal, gen
from gpaw.eigensolvers import RMMDIIS
from gpaw.cluster import Cluster
from gpaw.lrtddft import LrTDDFT
h = 0.3 # Gridspacing
e_singlet = 4.3
e_singlet_lr = 4.3
if setup_paths[0] != '.':
setup_paths.insert(0, '.')
gen('Na', xcname='PBE', scalarrel=True, exx=True, yukawa_gamma=0.40)
gen('Cl', xcname='PBE', scalarrel=True, exx=True, yukawa_gamma=0.40)
c = {'energy': 0.005, 'eigenstates': 1e2, 'density': 1e2}
mol = Cluster(molecule('NaCl'))
mol.minimal_box(5.0, h=h)
calc = GPAW(txt='NaCl.txt', xc='LCYPBE:omega=0.40:excitation=singlet',
eigensolver=RMMDIIS(), h=h, occupations=FermiDirac(width=0.0),
spinpol=False, convergence=c)
mol.set_calculator(calc)
mol.get_potential_energy()
(eps_homo, eps_lumo) = calc.get_homo_lumo()
e_ex = eps_lumo  eps_homo
equal(e_singlet, e_ex, 0.15)
calc.write('NaCl.gpw')
lr = LrTDDFT(calc, txt='LCY_TDDFT_NaCl.log', istart=6, jend=7, nspins=2)
lr.write('LCY_TDDFT_NaCl.ex.gz')
if world.rank == 0:
lr2 = LrTDDFT('LCY_TDDFT_NaCl.ex.gz')
lr2.diagonalize()
ex_lr = lr2[1].get_energy() * Hartree
equal(e_singlet_lr, e_singlet, 0.05)
Support for IVOs in lrTDDFT is done along the work of Berman and Kaldor [BK79].
If the number of bands in the calculation exceeds the number of bands delivered by the datasets, GPAW initializes the missing bands randomly. Calculations utilizing HartreeFock exchange can only use the RMMDIIS eigensolver. Therefore the states might not converge to the energetically lowest states. To circumvent this problem on can made a calculation using a semilocal functional like PBE and uses this wavefunctions as a basis for the following calculation utilizing HartreeFock exchange as shown in the following code snippet which uses PBE0 in conjuncture with the IVOs:
"""Test calculation for unoccupied states using IVOs."""
from __future__ import print_function
from ase.build import molecule
from gpaw.cluster import Cluster
from gpaw import GPAW, KohnShamConvergenceError, FermiDirac
from gpaw.eigensolvers import CG, RMMDIIS
calc_parms = [
{'xc': 'PBE0:unocc=True',
'eigensolver': RMMDIIS(niter=5),
'convergence': {
'energy': 0.005,
'bands': 2,
'eigenstates': 1e4,
'density': 1e3}},
{'xc': 'PBE0:excitation=singlet',
'convergence': {
'energy': 0.005,
'bands': 'occupied',
'eigenstates': 1e4,
'density': 1e3}}]
def calc_me(atoms, nbands):
"""Do the calculation."""
molecule_name = atoms.get_chemical_formula()
atoms.set_initial_magnetic_moments([1.0, 1.0])
fname = '.'.join([molecule_name, 'PBESIN'])
calc = GPAW(h=0.25,
xc='PBE',
eigensolver=CG(niter=5),
nbands=nbands,
txt=fname + '.log',
occupations=FermiDirac(0.0, fixmagmom=True),
convergence={
'energy': 0.005,
'bands': nbands,
'eigenstates': 1e4,
'density': 1e3})
atoms.set_calculator(calc)
try:
atoms.get_potential_energy()
except KohnShamConvergenceError:
pass
if calc.scf.converged:
for calcp in calc_parms:
calc.set(**calcp)
try:
calc.calculate(system_changes=[])
except KohnShamConvergenceError:
break
if calc.scf.converged:
calc.write(fname + '.gpw', mode='all')
loa = Cluster(molecule('NaCl'))
loa.minimal_box(border=6.0, h=0.25, multiple=16)
loa.center()
loa.translate([0.001, 0.002, 0.003])
nbands = 25
calc_me(loa, nbands)
Non selfconsistent planewave implementation¶
See this tutorial: PBE0 calculations for bulk silicon.

class
gpaw.xc.exx.
EXX
(calc, xc=None, kpts=None, bands=None, ecut=None, stencil=2, omega=None, world=<gpaw.mpi.SerialCommunicator object>, txt=<_io.TextIOWrapper name='<stdout>' mode='w' encoding='UTF8'>, timer=None)[source]¶ Non selfconsistent hybrid functional calculations.
Eigenvalues and total energy can be calculated.
 calc: str or PAW object
GPAW calculator object or filename of saved calculator object.
 xc: str
Name of functional. Use one of EXX, PBE0, HSE03, HSE06 or B3LYP. Default is EXX.
 kpts: list of in or list of list of int
List of indices of the IBZ kpoints to calculate the quasi particle energies for. Default is all kpoints. One can also specify the coordiantes of the kpoint. As an example, Gamma and X for an FCC crystal would be: kpts=[[0, 0, 0], [1 / 2, 1 / 2, 0]].
 bands: tuple of two ints
Range of band indices, like (n1, n2), to calculate the quasi particle energies for. Bands n where n1<=n<n2 will be calculated. Note that the second band index is not included. Default is all occupied bands.
 ecut: float
Plane wave cutoff energy in eV. Default it the same as was used for the groundstate calculations.
 AB98
C. Adamo and V. Barone. Toward Chemical Accuracy in the Computation of NMR Shieldings: The PBE0 Model.. Chem. Phys. Lett. 298.1 (11. Dec. 1998), S. 113–119.
 Ba94
V. Barone. Inclusion of Hartree–Fock exchange in density functional methods. Hyperfine structure of second row atoms and hydrides. Jour. Chem. Phys. 101.8 (1994), S. 6834–6838.
 BK79
M. Berman and U. Kaldor. Fast calculation of excitedstate potentials for raregas diatomic molecules: Ne2 and Ar2. Chem. Phys. 43.3 (1979), S. 375–383.
 HA71
S. Huzinaga and C. Arnau. Virtual Orbitals in Hartree–Fock Theory. II. Jour. Chem. Phys. 54.5 (1. Ma. 1971), S. 1948–1951.