INTERACTIONS subblock in QUILD block

One of the most important input-parts for multi-level jobs is the INTERACTIONS subblock of QUILD, where one should define how the different descriptions should be applied to the different regions. At the part where we explained the multi-level approaches, we already showed some examples of how to combine different methodologies. Below is another example input where all possible options are given.

QUILD
  INTERACTIONS
    TOTAL     description 1
    REPLACE   region 1 region 2    description 3 for description 2
    REPLACE   region 1             description 4 for description 3
    INTXN     region 1 region 2    description 3 for description 2
    S2CORR    region 1  spin-splus-description 2 for contaminated-description 1
  SUBEND
END

If an INTERACTIONS subblock is present (if none is present it means no multi-level setup is done, i.e. pure QM or MM), there should always be a line with the description of the total system, as shown in the first line of the INTERACTIONS subblock. Then if you want to replace the interactions for one (or more) region(s), you could do so as indicated in the second and third line. Finally, if you want to replace the interaction between two regions, as we need for DNA where we replace the BP86 π-stacking by LDA π-stacking, the fourth line of the INTERACTIONS subblock should be used. Finally, the last line can be used for spin-contamination corrections for one (or more) regions.

Note that in all cases it is not necessary at all to add the "region", "description" and "for" words in the INTERACTIONS subblock; they are ignored when reading the input. The program reads the line, uses the last two integers for the descriptions and the ones before for the regions. Therefore, a completely equivalent input would be as shown below. However, for better readability, it is to be advised to always use the additional text anyway.

QUILD
  INTERACTIONS
    TOTAL             1
    REPLACE   1 2   3 2
    REPLACE   1     4 3
    INTXN     1 2   3 2
    S2CORR    1     2 1
    S2CORR    1 3   2 1
  SUBEND
END

Note that in the last line, it is indicated that the spin-contamination correction is applied to regions 1 and 3 together.