Skip to content

NEVPT2 Calculation

NEVPT2

Dynamic correlation can be added on top of the CASSCF or MCSCF calculations via the strongly-contracted variant of N-electron valence state perturbation theory (SC-NEVPT2).1,2 It is invoked by setting the keyword DoNEVPT2 to true. The following is a sample input of a NEVPT2 calculation on top of a CASSCF calculation (CISolver FCI) for the Lithium Flouride Molecule:

fci-nevpt2.inp
 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
General
  CalcType CASSCF
  Charge 0
  Mult 1
  ORCAJSONName orca.json
    Ints RI
  Basis def2-svp
  AuxBasis def2-JK
End

CASSCF
  NEl 2
  NOrb 2
  NRoots 3
  CISolver FCI
  MaxIter 35
  OrbStep SuperCIPTDIIS
  SwitchOrbStep DIIS
  DoNEVPT2 true
  PTCanonStep SA
End

Geom
  Li 0.0 0.0 0.0
  F  0.0 0.0 5.0
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

QD-NEVPT2

Based on the work of Angeli et al3, the QD-NEVPT2 approach was also implemented. To conduct a QD-NEVPT2 calculation, set the DoQDNEVPT2 to true. As outlined in the original work, the effective Hamiltonian in this type of perturbation theory is not Hermitean. You can choose different variants of how the corresponding eigenvalue equation is set up and solved with the keyword QDNEVPT2Type (1).

  1. The options for QDNEVPT2Type are
    • VanVleck (default)
    • Cloizeaux
    • Bloch
qd-nevpt2.inp
 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
General
  CalcType CASSCF
  Charge 0
  Mult 1
  ORCAJSONName orca.json
  Ints RI
  Basis def2-svp
  AuxBasis def2-JK
End

CASSCF
  NEl 2
  NOrb 2
  NRoots 3
  CISolver FCI
  MaxIter 35
  OrbStep SuperCIPTDIIS
  SwitchOrbStep DIIS
  FullConvergence true
  DoNEVPT2 true
  DoQDNEVPT2 true
  QDNEVPT2Type VanVleck
  PTCanonStep SA
End

Geom
  Li 0.0 0.0 0.0
  F  0.0 0.0 5.0
End

EN-NEVPT2

The above NEVPT2 methods are available for selected CI calculations with the CFG-HCI method as well. However, for large active spaces, the regular SC-NEVPT2 method might become prohibitively expensive due to the presence of the so-called residual terms. As an alternative we have developed a method where the expensive \(\hat{V}^{-1}_a\) and \(\hat{V}^{+1}_i\) perturber functions are treated via an uncontracted Epstein-Nespet (EN-PT2) approach. We shall abbreviate this method as EN-NEVPT2. An example input is given below where EN-NEVPT2 calculation is enabled simply using the DoENEVPT2 keyword.

General
  CalcType CASSCF
  Charge 1
  Mult 2
  Ints RI
  Basis def2-SVP
  AuxBasis def2-JK
  OrbGuess inporbs.C0
End

CASSCF
  NEl 9
  NOrb 8
  NRoots 1
  CISolver HCI
  OrbStep FNR
  DoENEVPT2 True
End

Geom
C        -0.24442585958060   -0.00000000000006   -1.59845731550875
C        -1.08086569645337    0.00000000000005   -0.53980274844727
C         0.53105603337312    0.00000000000005   -0.49509905342901
Si       -0.32364392033916   -0.00000000000004    1.25326103938503
End

Note

The EN-NEVPT2 method is presently not implemented for QD-NEVPT2.

Note

The EN-NEVPT2 method is only available for HCI wave functions.

  1. C. Angeli, R. Cimiraglia, S. Evangelisti, T. Leininger, J. Malrieu, J. Chem. Phys. 2001, 114, 10252. ↩

  2. C. Angeli, R. Cimiraglia, J. Malrieu, J. Chem. Phys. 2002, 117, 9138. ↩

  3. C. Angeli, S. Borini, M. Cestari, R. Cimiraglia, J. Chem. Phys. 2004, 121, 4043–4049. ↩

  4. Y. Guo, K. Sivalingam, V.G. Chilkuri, F. Neese, J. Chem. Phys. 2025. 162, 14. ↩