Requests a density-functional-based tight-binding semi-empirical calculation, a method which is parametrized via the results of DFT calculations:
- DFTB uses the tabulated matrix elements as in the original implementation of Elstner and coworkers [Porezag95, Elstner98].
- DFTBA is a version that uses analytic expressions for the matrix elements rather than tabulated ones [Zheng07].
Energies, gradients, and frequencies.
Analytic second derivatives are available when the analytic DFTB parameters (provided with the program) are used, but are not available when the tabulated parameters from dftb.org are used. This is because the linear interpolation that is done using the tabulated parameters give discontinuous first derivatives at first points, so the second derivatives do not always exist. If you want to ignore this potential problem and compute second derivatives using the tabulated parameters, then Freq=Numer must be specified in the job’s route section.
DFTBA is parametrized for all pairs of H, C, N, and O. It is also parametrized for the metals Sc, Ti, Fe, Co, and Ni but only with H, C, N, and O. That is, Fe5CO and Sc5CO are supported, but Fe4ScCO is not.
The DFTB parameter files are copyright by Professor Elstner and must be obtained from him.
The following input file format runs a DFTBA calculation using the parameter set provided with Gaussian:
# DFTBA OPT FREQ Ala3 DFTB frequencies 0,1 C,0,-4.5929012011,1.0163256276,1.6498020765 O,0,-5.6641782096,0.9622594116,2.2369288649 H,0,-5.788876035,3.2375262156,-2.1703220199 N,0,-4.4446298947,1.4038535552,0.3517633631 Molecule specification continues … @GAUSS_EXEDIR:dftba.prm
For DFTB, the same format of parameter files is used as in other programs: one file for each pair of elements, with the order of the two elements being significant. Accordingly, a calculation on H2CO would use a parameter input section something like this:
@cc.prm @oo.prm @hh.prm @co.prm @oc.prm @ch.prm @hc.prm @oh.prm @ho.prm
The energy appears in the output as follows:
SCF Done: E(RDFT-SCTBA) = -33.9465130617 A.U. after 11 cycles
Modifying Slater-Koster Files
DFTB Parameter (.skf) Files and Gaussian
The handling of DFTB input files has been modified for compatibility with the files provided by Elstner: HTML data at the end of the file is ignored. Also, multipliers—e.g. 10*1.0—are now accepted.
Modifying Slater-Koster files (.skf) from dftb.org for use with Gaussian
The first line of each file must be edited to identify the two elements involved. For example, in the file H-S.skf the first line should contain the atomic numbers, so it goes from dftb.org format:
to Gaussian format:
0.02, 500 1 16 Add atomic numbers for hydrogen and sulfur.
The second field in the first line of each file should contain the number of lines in the file containing grid points for the Hamiltonian and overlap integrals. However, these values are often just placeholders of 500 (as above) or 600. Many of the files—especially older ones—actually supply some other number of points. If you count the number of lines between the one with the first point and the last line before Spline, this will yield the required value:
2.000000000000E-02, 601 1 6 The next line is line 1. 0.000000000000E+00 0.000000000000E+00 0.000000000000E+00 0.000000000000E+00 0.000000000000E+00 0.000000000000E+00 0.000000000000E+00 0.000000000000E+00 -1.668269422592E-05 1.066481969438E-06 0.000000000000E+00 0.000000000000E+00 … 0.000000000000E+00 0.000000000000E+00 0.000000000000E+00 0.000000000000E+00 0.000000000000E+00 0.000000000000E+00 4.881740699644E-05 -7.526317479277E-06 Spline The preceding line is the last one to count. 30 3.5
You would then edit the first line to reflect the actual number of points:
0.02, 601 1 16
Some of the files involving hydrogens are links rather than separate files because they are the same as the corresponding file with the order of the elements reversed. For example, H-C.skf is a link to C-H.skf. However, Gaussian expects both files to be provided, differing only in the order of the atomic numbers on the first line.
Last updated on: 05 January 2017. [G16 Rev. B.01]