ASS1ST Procedure¶

The Active Space Selection based on 1^st order perturbation theory (ASS1ST) procedure allows for a chemically motivated and physically sound choice of active orbitals for any given molecular system.¹^,² The ASS1ST procedure relies on a set of quasi-natural orbital orbitals constructed from the unrelaxed 1^st order SC-NEVPT wavefunction and their corresponding natural orbital occupation numbers (NOONs). To invoke the computation of the quasi-natural orbitals and their NOONs the DoASSIST keyword in the CASSCF block has to be set to true as shown below:

ASS1ST.inp
General
  CalcType CASSCF
  Ints RI
  Basis def2-svp
  AuxBasis def2-svp-c
  Charge 0
  Mult 1
  OrcaJSONNAME orca.json
End

CASSCF
  NOrb 5
  NEl 6
  CISolver FCI
  OrbStep FNR
  TrustRadius 0.1
  DoASSIST True
  IPEAShift True
  # IPEAShift False(Default)
  # IPEAShiftValue 0.25(Default)
  IPEAShiftValue 0.25
End

Geom
  Li 0.0 0.0 0.0
  F  0.0 0.0 5.0
End

Note, that the first-order interacting space density is calculated separately for the internal-internal and the external-external subblocks of the unrelaxed density matrix. The NOONs are printed in the output as shown below:

ASS1ST.out

================================================
NEVPT2 QUASI-NATURAL ORBITAL OCCUPATION NUMBERS
================================================

NO.: OCC NO
–––––––
Internal
–––––––
0: 2.000000
1: 1.999950
2: 1.999873
3: 1.999872
      .
      .
      .
47: 1.958529
48: 1.947980
49: 1.939802
–––––––
Active
–––––––
50: 1.770270
51: 1.360853
52: 1.342410
53: 0.963549
54: 0.562918
–––––––
External
–––––––
55: 0.058295
56: 0.045937
57: 0.043523
58: 0.035375
59: 0.032443
      .
      .
      .

The natural orbitals are stored in the file calcname.C0_FOIS. Furthermore, they are stored in an ORCA readable format in the file calcname.C0_FOIS.orca. Note, in order to read these files into ORCA you have to invoke the !ReadCFile command. The occupation numbers of the generated quasi-natural orbitals prove to be an objective measure for selecting a reasonable set of active orbitals. For illustrative examples of ASS1ST procedure we refer the reader to the relevant literature.¹^,²^,³

An important point to note is that in order to capture strong correlation small basis sets are usually sufficient. In our hands the usage of double-zeta basis sets yielded satisfactory results in terms of active space choices. The resulting orbitals can then be used as starting orbitals for subsequent calculations that feature larger basis sets and take into account dynamic electron correlation to obtain accurate results.

A. Khedkar, M. Roemelt, J. Chem. Theory Comput. 2019, 15, 3522–3536. ↩↩
A. Khedkar, M. Roemelt, J. Chem. Theory Comput. 2020, 16, 4993–5005. ↩↩
A. Khedkar, M. Roemelt, Phys. Chem. Chem. Phys. 2020, 22, 17677–17686. ↩