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Selective CI Calculation

As an alternative to, a configuration-based heatbath CI (HCI) has been implemented in the program.1,2 HCI can be used for both providing energies with near-FCI accuracy in benchmark studies or for performing approximate CASSCF calculations with large active spaces. In the latter case it should be noted, that the orbital optimization is carried out with respect to the variational energy only. Below is an example input of an approximate CASCI calculation done with HCI.

hci.inp
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General
  CalcType CASSCF
  Ints RI
  Basis cc-pvdz
  Mult 1
  Charge 0
  OrbGuessName inporbs.C0
End

CASSCF
  NEl 12
  NOrb 12
  NRoots 1
  CISolver HCI
  OrbStep FNR  
End

CI
  DoHCIPT2 True
  EpsilonGen 1e-2
  EpsilonHCI 1e-5
  EpsilonHCIPT2 1e-6
End

Geom
  C -0.496835372 -0.000000687 -3.662272380
  C -0.496835388 -0.000000950 -5.054272380
  N -0.496835236  0.000000831 -6.217272380
  N -0.496835242  0.000000735 -2.499272380
End

It should be noted that the PT2-correction is only calculated once at the end of the CASSCF calculation and it has no influence on the orbital optimization. The DoHCIPT2 flag is by default set to true, but it is most appropriate when near-exact CAS energies are wanted. Otherwise it may act as an indicator showing how accurate the selective-CI calculation (variational part) is. In common large-scale CASSCF calculations, it is recommended to set the flag DoHCIPT2to false. Additionally, if the CIPSI-pruning is not wanted, the corresponding flag DoCIPSIOnHCI can be set to false – this will yield a bit lower energy, but also a less compact wave function. By default CIPSI-pruning is enabled and recommended, especially in larger CAS calculations.

  1. M. Ugandi, M. Roemelt, J. Comput. Chem. 2023, 44, 2374–2390, DOI 10.1002/jcc.27203 ↩

  2. A. A. Holmes, H. J. Changlani, C. J. Umrigar, J. Chem. Theory Comput. 2016, 12,1561–1571. ↩