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orbital strings

Guest on 19th May 2022 03:27:23 PM

  1. :orphan:
  2.  
  3.  
  4. .. _orbital_strings:
  5.  
  6. Specification of orbital strings
  7. ================================
  8.  
  9. A subset of orbitals can be specified in two ways. Usually it suffices
  10. to specify the keyword energy, followed by three real numbers to specify
  11. a lower limit, upper limit and quasi-degeneracy threshold. All orbitals
  12. that have an energy between the two thresholds are then selected. The
  13. quasi-degeneracy threshold is used to extend the range if the lower or
  14. upper threshold cuts through a set of closely energy-spaced orbitals. It
  15. will move the upper or lower limit until it encounters a large enough
  16. gap between orbital energies. The default value used in the AO-MO
  17. integral transformation program to specify the active space for
  18. correlated calculations is e.g. given by the string::
  19.  
  20.   energy -10.0 20.0 1.0
  21.  
  22. This picks out all orbitals that have an energy between -10.0 and +20.0
  23. hartree (a.u.), with a minimum gap of 1.0 hartree.
  24.  
  25. More detailed control is possible by using an orbital string. Here a
  26. character string is given where individual orbitals are listed separated
  27. by a comma or as a range of orbitals. The range limit is then separated
  28. by "..". Positive energy and negative energy orbitals are indicated
  29. using positive and negative numbers, respectively. For instance the
  30. orbital string::
  31.  
  32.   -5..5,7
  33.  
  34. This picks out the five upper negative energy ("positronic") and five lower
  35. positive energy ("electronic") orbitals plus positive energy orbital number
  36. seven.
  37.  
  38. If all orbitals (both positive and negative energy) are wanted, then one may
  39. write::
  40.  
  41.   all
  42.  
  43. The concept of orbitals strings gives great flexibility in the specification of
  44. orbitals and serves as an alternative to editing of vector files. A drawback
  45. is, however, that one needs to count the number of orbitals manually, which may
  46. become cumbersome for cases with large ranges of virtual orbitals.

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