Q-Chem 4.4 User’s Manual

10.18 Quantum Transport Properties via the Landauer Approximation

Quantum transport at the molecule level involves bridging two electrodes with a molecule or molecular system, and calculating the properties of the resulting molecular electronic device, including current-voltage curves, the effect of the electrodes on the molecular states, etc. For a general introduction to the field, the following references are useful [639, 640].

The quantum transport code in Q-Chem is developed by Prof. Barry Dunietz (Kent State) and his group. This package is invoked by the $rem variable TRANS_ENABLE.


Decide whether or not to enable the molecular transport code.





Do not perform transport calculations.



Perform transport calculations in the Landauer approximation.


Print matrices for subsequent calls for tranchem.exe as a stand-alone post-processing


utility, or for generating bulk model files.


Use as required.

Output is provided in the Q-Chem output file and in the following additional files:

T-Chem requires two parameter files:

All the numbers must be provided. The details of file formats are given in Table 10.3 and Table 10.4.

Input example


142 Totorb

total number of atomic orbitals in the cluster model of the molecular junction

22 lob

number of AO’s representing the left electrode (the first lob functions of input)

22 rob

number of AO’s representing the right electrode (the last rob functions of input)

22 lg

size of the repeating unit of the left electrode

22 rg

size of the repeating unit of the right electrode

22 ql

must be set to the lg value (later used in NEGF optimizations)

22 qr

must be set to the rg value (later used in NEGF optimizations)

Table 10.3: Example of a Trans-model.para file, and the meaning of each input line. All lines must be present

Key points to note about the Trans-model.para file, as documented in Table 10.3:

Input example

Keywords included on each line





-6.5 -8.5 -4.5 300


0 0 0.01 0.01 0.07


0 1 1


4.0 4 100 1.0


1 2 100




Table 10.4: Example of a Trans-method.para file, and the keywords that correspond to each input line. The supported values of each keyword is listed below. All lines must be present.

The allowed values of the various keywords that comprise each line of the Trans-method.para file follow.

As an example, the sample Q-Chem input that works with the files documented in Table 10.3 and Table 10.4 is given below.

Example 10.244  Quantum transport calculation applied to C$_6$ between two gold electrodes.

  0 1
  Au -0.2 0 0
  Au  2.5 0 0
  C   4.8 0 0
  C   6.5 0 0 
  C   8.2 0 0
  C   9.9 0 0
  C  11.6 0 0
  C  13.3 0 0
  Au 15.6 0 0
  Au 18.3 0 0

  jobtype               SP
  method                B3LYP
  BASIS                 lanl2dz
  ECP                   lanl2dz
  GEOM_OPT_MAXCYC       200
  INCDFT                FALSE
  mem_static            8000
  max_scf_cycles        400
  MEM_TOTAL             32000
  scf_convergence       10
  scf_algorithm         diis
  trans_enable          1