# Single-Point Energy — `energy()`

- Psi4 Native Energy Methods
- Psi4 Native DFT Energy Methods
- MRCC Interfaced Energy Methods
- CFOUR Interfaced Energy Methods

`psi4.``energy`(name[, molecule, return_wfn, restart_file])

  Function to compute the single-point electronic energy.

  Returns: | float – Total electronic energy in Hartrees. SAPT & EFP return interaction energy.
  Returns: | (float, `Wavefunction`) – energy and wavefunction when return_wfn specified.
  PSI variables: |

  - `CURRENT ENERGY`
  - `CURRENT REFERENCE ENERGY`
  - `CURRENT CORRELATION ENERGY`

  Parameters:

  - name (string) –

    `'scf'` || `'mp2'` || `'ci5'` || etc.

    First argument, usually unlabeled. Indicates the computational method to be applied to the system.

  - molecule (molecule) –

    `h2o` || etc.

    The target molecule, if not the last molecule defined.

  - return_wfn (boolean) –

    `'on'` || \(\Rightarrow\) `'off'` \(\Leftarrow\)

    Indicate to additionally return the `Wavefunction` calculation result as the second element (after float energy) of a tuple.

  - restart_file (string) –

    `['file.1, file.32]` || `./file` || etc.

    Binary data files to be renamed for calculation restart.

  name | calls method
  --- | ---
  efp | effective fragment potential (EFP) [manual]
  scf | Hartree–Fock (HF) or density functional theory (DFT) [manual]
  hf | HF self consistent field (SCF) [manual]
  hf3c | HF with dispersion, BSSE, and basis set corrections [manual]
  pbeh3c | PBEh with dispersion, BSSE, and basis set corrections [manual]
  dcft | density cumulant functional theory [manual]
  mp2 | 2nd-order Møller–Plesset perturbation theory (MP2) [manual] [details]
  mp3 | 3rd-order Møller–Plesset perturbation theory (MP3) [manual] [details]
  fno-mp3 | MP3 with frozen natural orbitals [manual]
  mp2.5 | average of MP2 and MP3 [manual] [details]
  mp4(sdq) | 4th-order MP perturbation theory (MP4) less triples [manual]
  fno-mp4(sdq) | MP4 (less triples) with frozen natural orbitals [manual]
  mp4 | full MP4 [manual] [details]
  fno-mp4 | full MP4 with frozen natural orbitals [manual]
  mpn | nth-order Møller–Plesset (MP) perturbation theory [manual]
  zaptn | nth-order z-averaged perturbation theory (ZAPT) [manual]
  omp2 | orbital-optimized second-order MP perturbation theory [manual]
  scs-omp2 | spin-component scaled OMP2 [manual]
  scs(n)-omp2 | a special version of SCS-OMP2 for nucleobase interactions [manual]
  scs-omp2-vdw | a special version of SCS-OMP2 (from ethene dimers) [manual]
  sos-omp2 | spin-opposite scaled OMP2 [manual]
  sos-pi-omp2 | A special version of SOS-OMP2 for pi systems [manual]
  omp3 | orbital-optimized third-order MP perturbation theory [manual]
  scs-omp3 | spin-component scaled OMP3 [manual]
  scs(n)-omp3 | a special version of SCS-OMP3 for nucleobase interactions [manual]
  scs-omp3-vdw | a special version of SCS-OMP3 (from ethene dimers) [manual]
  sos-omp3 | spin-opposite scaled OMP3 [manual]
  sos-pi-omp3 | A special version of SOS-OMP3 for pi systems [manual]
  omp2.5 | orbital-optimized MP2.5 [manual]
  lccsd, cepa(0) | coupled electron pair approximation variant 0 [manual] [details]
  fno-lccsd, fno-cepa(0) | CEPA(0) with frozen natural orbitals [manual]
  cepa(1) | coupled electron pair approximation variant 1 [manual]
  fno-cepa(1) | CEPA(1) with frozen natural orbitals [manual]
  cepa(3) | coupled electron pair approximation variant 3 [manual]
  fno-cepa(3) | CEPA(3) with frozen natural orbitals [manual]
  acpf | averaged coupled-pair functional [manual]
  fno-acpf | ACPF with frozen natural orbitals [manual]
  aqcc | averaged quadratic coupled cluster [manual]
  fno-aqcc | AQCC with frozen natural orbitals [manual]
  qcisd | quadratic CI singles doubles (QCISD) [manual]
  fno-qcisd | QCISD with frozen natural orbitals [manual]
  lccd | Linear CCD [manual] [details]
  fno-lccd | LCCD with frozen natural orbitals [manual]
  olccd | orbital optimized LCCD [manual]
  cc2 | approximate coupled cluster singles and doubles (CC2) [manual]
  ccd | coupled cluster doubles (CCD) [manual]
  ccsd | coupled cluster singles and doubles (CCSD) [manual] [details]
  bccd | Brueckner coupled cluster doubles (BCCD) [manual]
  fno-ccsd | CCSD with frozen natural orbitals [manual]
  qcisd(t) | QCISD with perturbative triples [manual]
  fno-qcisd(t) | QCISD(T) with frozen natural orbitals [manual]
  ccsd(t) | CCSD with perturbative triples (CCSD(T)) [manual] [details]
  ccsd(at) | CCSD with asymmetric perturbative triples (CCSD(AT)) [manual] [details]
  bccd(t) | BCCD with perturbative triples [manual]
  fno-ccsd(t) | CCSD(T) with frozen natural orbitals [manual]
  cc3 | approximate CC singles, doubles, and triples (CC3) [manual]
  ccenergy | expert full control over ccenergy module
  dfocc | expert full control over dfocc module
  cisd | configuration interaction (CI) singles and doubles (CISD) [manual] [details]
  fno-cisd | CISD with frozen natural orbitals [manual]
  cisdt | CI singles, doubles, and triples (CISDT) [manual]
  cisdtq | CI singles, doubles, triples, and quadruples (CISDTQ) [manual]
  cin | nth-order CI [manual]
  fci | full configuration interaction (FCI) [manual]
  detci | expert full control over detci module
  casscf | complete active space self consistent field (CASSCF) [manual]
  rasscf | restricted active space self consistent field (RASSCF) [manual]
  mcscf | multiconfigurational self consistent field (SCF) [manual]
  psimrcc | Mukherjee multireference coupled cluster (Mk-MRCC) [manual]
  dmrg-scf | density matrix renormalization group SCF [manual]
  dmrg-caspt2 | density matrix renormalization group CASPT2 [manual]
  dmrg-ci | density matrix renormalization group CI [manual]
  sapt0 | 0th-order symmetry adapted perturbation theory (SAPT) [manual]
  ssapt0 | 0th-order SAPT with special exchange scaling [manual]
  fisapt0 | 0th-order functional and/or intramolecular SAPT [manual]
  sapt2 | 2nd-order SAPT, traditional definition [manual]
  sapt2+ | SAPT including all 2nd-order terms [manual]
  sapt2+(3) | SAPT including perturbative triples [manual]
  sapt2+3 | SAPT including all 3rd-order terms [manual]
  sapt2+(ccd) | SAPT2+ with CC-based dispersion [manual]
  sapt2+(3)(ccd) | SAPT2+(3) with CC-based dispersion [manual]
  sapt2+3(ccd) | SAPT2+3 with CC-based dispersion [manual]
  sapt2+dmp2 | SAPT including all 2nd-order terms and MP2 correction [manual]
  sapt2+(3)dmp2 | SAPT including perturbative triples and MP2 correction [manual]
  sapt2+3dmp2 | SAPT including all 3rd-order terms and MP2 correction [manual]
  sapt2+(ccd)dmp2 | SAPT2+ with CC-based dispersion and MP2 correction [manual]
  sapt2+(3)(ccd)dmp2 | SAPT2+(3) with CC-based dispersion and MP2 correction [manual]
  sapt2+3(ccd)dmp2 | SAPT2+3 with CC-based dispersion and MP2 correction [manual]
  sapt0-ct | 0th-order SAPT plus charge transfer (CT) calculation [manual]
  sapt2-ct | SAPT2 plus CT [manual]
  sapt2+-ct | SAPT2+ plus CT [manual]
  sapt2+(3)-ct | SAPT2+(3) plus CT [manual]
  sapt2+3-ct | SAPT2+3 plus CT [manual]
  sapt2+(ccd)-ct | SAPT2+(CCD) plus CT [manual]
  sapt2+(3)(ccd)-ct | SAPT2+(3)(CCD) plus CT [manual]
  sapt2+3(ccd)-ct | SAPT2+3(CCD) plus CT [manual]
  adc | 2nd-order algebraic diagrammatic construction (ADC) [manual]
  eom-cc2 | EOM-CC2 [manual]
  eom-ccsd | equation of motion (EOM) CCSD [manual]
  eom-cc3 | EOM-CC3 [manual]

  Examples: |

      >>> # [1] Coupled-cluster singles and doubles calculation with psi code
      >>> energy('ccsd')

      >>> # [2] Charge-transfer SAPT calculation with scf projection from small into
      >>> #     requested basis, with specified projection fitting basis
      >>> set basis_guess true
      >>> set df_basis_guess jun-cc-pVDZ-JKFIT
      >>> energy('sapt0-ct')

      >>> # [3] Arbitrary-order MPn calculation
      >>> energy('mp7')

      >>> # [4] Converge scf as singlet, then run detci as triplet upon singlet reference
      >>> # Note that the integral transformation is not done automatically when detci is run in a separate step.
      >>> molecule H2 {\n0 1\nH\nH 1 0.74\n}
      >>> set basis cc-pVDZ
      >>> set reference rohf
      >>> scf_e, scf_wfn = energy('scf', return_wfn=True)
      >>> H2.set_multiplicity(3)
      >>> core.MintsHelper(scf_wfn.basisset()).integrals()
      >>> energy('detci', ref_wfn=scf_wfn)

      >>> # [5] Run two CI calculations, keeping the integrals generated in the first one.
      >>> molecule ne {\nNe\n}
      >>> set basis cc-pVDZ
      >>> cisd_e, cisd_wfn = energy('cisd', return_wfn=True)
      >>> energy('fci', ref_wfn=cisd_wfn)

      >>> # [6] Can automatically perform complete basis set extrapolations
      >>> energy("CCSD/cc-pV[DT]Z")

      >>> # [7] Can automatically perform delta corrections that include extrapolations
      >>> # even with a user-defined extrapolation formula. See sample inputs named
      >>> # cbs-xtpl* for more examples of this input style
      >>> energy("MP2/aug-cc-pv([d,t]+d)z + d:ccsd(t)/cc-pvdz", corl_scheme=myxtplfn_2)