Remove Fragment Orbitals

By default all fragment orbitals (the MOs of the fragment computation), which are stored on the fragment file, are used as basis functions for the overall molecule, see Chapter 1.2. You can remove one or more of these fragment orbitals from the basis set of the molecule. This may be useful for special analyzes, for instance to study the effect of deleting all virtual MOs of a particular fragment (CSOV analysis, Constrained Space Orbital Variation). It may also enhance the efficiency since you effectively reduce the size of the basis set, but you should be aware of the potential effects on the results.

REMOVEFRAGORBITALS
 fragtype
   subspecies nremove
   subspecies nremove
   ...
 subend
 fragtype
   subspecies nremove
   ...
 subend
 ....
 (etc.)
 ....
end

fragtype

One of the fragment types in the system. Any subset of the available fragment types can be used here as subkey. The subkeys are block type keys; their data blocks end subend.

subspecies

One of the subspecies of the irreducible representations of the point group symmetry that was used in the calculation of the fragment itself. This requires of course that one knows the symmetry that has been used for the fragment calculation.

nremove

The number of fragment orbitals of the pertaining representation that will not be used as basis functions for the overall system. The highest (in energy eigenvalue) nremove orbitals are discarded. You must not remove occupied fragment orbitals.

By default (omission of the key) all fragment orbitals are used in the basis set for the system.

Important Note

It is imperative that any removal of fragment orbitals will not break the symmetry of the molecule. This consideration is relevant when for instance two different subspecies of a fragment irrep contribute to different partner subspecies in one of the irreps of the molecule. In such a case, when one removes an orbital in such a fragment subspecies, its partner orbital should also be removed. If this is violated an error may occur or the results will simply be wrong. Quite likely, the program will detect the error, but this may occur only in the final (analysis) stage of the calculation so that a lot of CPU time may have been wasted.

Example: consider a single-atom fragment, computed in atom symmetry, used as fragment in a c(lin) molecule and assume that the p:x and p:y fragment orbitals contribute to respectively the pi:x and pi:y subspecies of the molecule. Then, when you remove one or more p:x fragment orbitals, you should also remove the same number of p:y fragment orbitals. Practical cases may be more complicated and whenever you use this key, make sure that you've fully analyzed and understood how the fragment irreps combine into the molecular symmetry representations. Hint: run the molecule, without removing any fragment orbitals, and stop at an early stage after the program has computed and printed the build-up of the molecular SFOs from the fragment orbitals. To control early aborts via input, use the key STOPAFTER.