# N-electron valence state perturbation theory (NEVPT2)

## Description

Calculations using the strongly contracted state-specific n-electron valence state perturbation theory (NEVPT2) are done using the keyword `"title" : "nevpt2"`.

## Keywords

The default values are recommended unless mentioned otherwise.

`frozen`

Description: to have frozen orbitals or not
Datatype: bool
Default: true.

`ncore`

Description: manually specify number of frozen orbitals, used when ‘frozen’ is turned on with `"frozen" : "true"`.
Datatype: int

`aux_basis`

Description: specify an auxiliary basis set for MP2
Datatype: string
Default: use the same density fitting basis as in Required keywords
Recommendation: use MP2-fit auxiliary basis (auxiliary basis ends with ‘ri’)

`istate`

Description: specific state used in the evaluation of the dynamical correlation
Datatype: int
Default: 0 (ground state)

## Example

    { "bagel" : [

    {
      "title" : "molecule",
      "basis" : "svp",
      "df_basis" : "svp-jkfit",
      "angstrom" : true,
      "geometry" : [
        { "atom" : "O",  "xyz" : [ 0.00000000000, 0.0,  0.000000000000]},
        { "atom" : "H",  "xyz" : [ 1.45860189536, 0.0,  0.504283963824]},
        { "atom" : "H",  "xyz" : [ 0.75860194558, 0.0, -0.504283963824]}
      ]
    },

    {
      "title" : "nevpt2",
      "nact" : 2,
      "nclosed" : 4,
      "frozen" : true,
      "thresh" : 1.0e-8,
      "thresh_scf" : 1.0e-8,
      "thresh_fci" : 1.0e-10
    }

    ]}

from which one obtains

    === DF-NEVPT2 calculation ===

         - RDMs, hole RDMs, others                   0.00
         - Fock computation                          0.01
      * 3-index integral transformation done
         - K matrices                                0.00
         - A, B, C, and D matrices                   0.00
      * ncache = 20
      * assembly done

        NEVPT2 correlation energy:   -0.1703158878      0.02

            (+0)     -0.0977514346
            (+0)'    -0.0257993320
            (+1)     -0.0086109214
            (+1)'    -0.0000000000
            (+2)     -0.0007336183
            (-1)     -0.0341475759
            (-1)'     0.0000000000
            (-2)     -0.0032730057

        NEVPT2 total energy:        -76.0205249507

## References

### BAGEL references

Description of Reference | Reference
--- | ---
Relativistic implementation of NEVPT2 in BAGEL | T. Shiozaki and W. Mizukami, J. Chem. Theory Comput. 11, 4733 (2015).

### General references

Description of Reference | Reference
--- | ---
Original reference for NEVPT2 | C. Angeli, R. Cimiraglia, S. Evangelisti, T. Leininger, and J.-P. Malrieu, J. Chem. Phys. 114, 10252 (2001).
Spin-free formulation | C. Angeli, R. Cimiraglia, and J.-P. Malrieu, J. Chem. Phys. 117, 9138 (2002).