DFTB+ is a program that implements a variety of density-functional tight binding methods. The DFTB wavefunction calculated by DFTB+ has an linear combination of atomic orbitals (LCAO) expression using Slater-type orbitals. For instructions on setting up the program as well as the different calculation levels and how to run a calculation, please refer to the package documentation.
Reading and Writing Structures
Critic2 understands molecular and crystal geometries read and written
by DFTB+, in its
.gen format. A gen file looks like this:
16 F C O N H 1 1 0.000000000000 0.500000000000 0.326000000000 [...] 0.000000000000 0.000000000000 0.000000000000 5.565000000001 0.000000000000 0.000000000000 0.000000000000 5.565000000001 0.000000000000 0.000000000000 0.000000000000 4.684000000002
where the system is treated as a molecule or a crystal depending on
whether the final four lines (the lattice vectors) are present. A
gen file can be used as input for DFTB+ and it can also be written
during a geometry optimization using the
OutputPrefix keyword in
Driver. On the other hand, critic2 can also write arbitrary
crystal structures in gen format:
crystal library urea write urea.gen
as well as molecular structures:
molecule library benzene write benzene.gen
This is useful for setting up new DFTB+ calculations.
Reading DFTB+ Wavefunctions
Three files are required to read a wavefunction calculated by DFTB+ files:
detailed.xmlfile, which is written by activating the
WriteDetailedXMLkeyword in the DFTB+ input.
eigenvec.binfile, which is written by DFTB+ when the
WriteEigenVectorskeyword is used.
.hsdfile containing the STO coefficients corresponding to the parametrization used in the calculation. These are provided by the DFTB+ authors and can be obtained online.
The kinetic energy density can be calculated from a DFTB+ wavefunction.
Two different types of systems can be run in DFTB+: crystals under periodic boundary conditions, and molecules in the gas-phase. In the first case, if using k-points other than Gamma, the wavefunction is complex, and the evaluation of the density is relatively slow. Molecules use real wavefunctions and they tend to be less crowded than crystals, so using molecular DFTB fields should be considerably faster.
Two complete examples are given in the package below: a molecule
(benzene) and a solid (graphite). To run them, execute
dftb+ in the
corresponding directories. Output files are already provided.
A critic2 example input for reading the DFTB+ structure and wavefunction of a molecule is:
# Read the benzene molecule from the gen file molecule benzene.gen 2 # In DFTB+, only valence electrons are used. Hence, we need core-augmentation # to account for the missing core density. Set ZPSP to the number # of valence electrons associated with a given atom (4 for carbon, # 5 for nitrogen, 6 for oxygen,...) zpsp c 4 # Load the DFTB+ wavefunction load detailed.xml eigenvec.bin ../3ob-3-1/wfc.3ob-3-1.hsd core # Find the critical points auto # Write a vmd file with the critical points cpreport benzene.vmd cell molcell graph # Calculate the electron density on a plane plane 0 -3 -3 0 3 -3 0 -3 3 101 101 contour log 41 relief 0 1
For a crystal:
# Read the graphite crystal from the gen file crystal graphite.gen # In DFTB+, only valence electrons are used. Hence, we need core-augmentation # to account for the missing core density. Set ZPSP to the number # of valence electrons associated with a given atom (4 for carbon, # 5 for nitrogen, 6 for oxygen,...) zpsp c 4 # Load the DFTB+ wavefunction load detailed.xml eigenvec.bin ../3ob-3-1/wfc.3ob-3-1.hsd core # Calculate the electron density on a plane plane 0 0 1/4 1 0 1/4 0 1 1/4 101 101 contour log 41 file plane
Example Files Package