the following is from chatGPT, what do you guys think ?
When using the 6-311G basis set, it is common to add diffuse functions to improve the description of electron correlation and dispersion interactions. One commonly used set of diffuse functions is the "polarization-consistent basis set" (pcS-n, where n is an integer), which adds diffuse functions to the valence and polarization sets in a consistent way.
For Fe2+ ferrous ion, a suitable choice of diffuse functions would depend on the level of accuracy required and the size of the system. As a starting point, you could consider adding diffuse functions up to the pcS-4 level, which would add 4 sets of diffuse functions to the 6-311G basis set.
To specify the pcS-4 basis set in Gaussian, you can use the following basis set specification:
2 3
Fe 2.74538330 8.28679554 5.00000000
O 4.55208397 8.06717607 5.00000000
C 5.30819317 9.07309328 5.00000000
O 5.97838127 9.96470142 5.00000000
1 2 1.0
2 3 2.0
3 4 3.0
4
Fe S 4
6-311G(d,p) Fe 4,4,4,4,4,4
****
C S 1
6-311G(d,p) C 3,3,3,3,3,3
****
O S 2
6-311G(d,p) O 3,3,3,3,3,3
****
```
In this input file, the "S" keyword specifies that diffuse functions should be added to the basis set, and the "4" after "Fe S" indicates that 4 sets of diffuse functions should be added to the iron atom. The same format is used for the carbon and oxygen atoms.
pc系列基组提出者Frank Jensen回复：It is difficult to find anything correct in the free flowing text below.
