Gaussian input file example

Natural transition orbitals NTOs can be a helpful way of obtaining a qualitative description of electronic excitations. They do so by transforming the ordinary orbital representation into a more compact form in which each excited state is expressed as a single pair of orbitals if possible : the NTO transition occurs from excited particle occupied to the empty hole unoccupied.

See [ Martin03 ] for a detailed description of natural transition orbitals. Here is an example input file for the first step. It is a TD-DFT calculation on a molecular structure that we have previously optimized and verified as a minimum:. Now we need to run a second calculation to generate and save the NTOs for visualization in GaussView or another graphics package. In this example, we compute the NTOs for the third excited state:. GaussView and other graphics packages will visualize whatever orbitals are present in the checkpoint file, so no special handling is required inside the visualization program.

Note that we use a separate checkpoint file for the NTOs. Before specifying the structure of the system, the overall charge of the system and the overall spin multiplicity must be given. The spin multiplicity "1" describes a singlet state, "2" a doublet state, "3" a triplet state.

Beware of additional blank lines in this case! In the current example the structure of formaldehyde is described in an internal coordinate system on 11 lines. A series of separate calculations can be combined into one input file by separating the various input sections with the Link1 comand. Please observe that the geometry of the system is only given explicitly for the first of these calculations. The keyword line can actually be longer than one physical line and is concluded by one additional blank line.

Following the blank line starts the comments section in free format providing space for any type of comment. It is good practice to include the name of the input file also here together with a short description of the job. The comments section is concluded by one additional blank line. The spin multiplicity "1" describes a singlet state, "2" a doublet state, "3" a triplet state. Beware of additional blank lines in this case!