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AC0 Calculation

Dynamic correlation can be added on top of the CASSCF or MCSCF calculations via the linearized variant of the adiabatic connection approximation (AC0).1 It is invoked by setting the keyword DoAC0 to true. The following is a sample input of a AC0 calculation on top of a CASSCF calculation (CISolver FCI) for the propenyl radical. To run this calculation you need to read in orbitals from a preceeding orca calculation ac0_sa_orca.json.

ac0_sa.inp
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General
  CalcType CASSCF
  Charge 0
  Mult 2
  ORCAJSONName ac0_sa_orca.json
  Basis def2-svp
End

CASSCF
  NEl 3
  NOrb 3
  NRoots 3
  CISolver FCI
  DoAC0 true
  PTCanonStep SA
  OrbStep FNR
  FullConvergence true
End

Geom
  C           2.81623297231720     -0.26561230779992      0.00819931420184
  C           3.98490958981717      0.47464708010529      0.01249731407978
  C           1.54227948243064      0.27346531676351     -0.00492809329372
  H           2.90766183347182     -1.34954832742008     -0.00274817369111
  H           4.95709022767437     -0.00152429282177     -0.00056556996781
  H           3.95873015158816      1.55937910533832      0.00068056283730
  H           1.39099562287169      1.34763580750932      0.00028705176666
  H           0.66170011982890     -0.35644238167468     -0.01342240593293
End

Info

In this example, state-averaged orbitals and their energies are used by setting the keyword PTCanonStep to SA(1).

  1. The PTCanonStep keyword can be set up with following string or integer.
    • State-Averaged Orbitals: SA or 0
    • State-Specific Orbitals: SS or 1

  1. Y. Guo, K. Pernal, Faraday Discuss. 2024, 254, 332–358. ↩