The **quantum chemical method** is specified via

**CALCULATION=method**

with the following choices for **method**

* I) Hartree-Fock Self-Consistent Field calculations*

**HF**(alternatively also via **SCF**)

* II) MÃ¸ller-Plesset Perturbation Theory (Many-Body Perturbation Theory)*

**MP2** (alternatively also via **MBPT(2)**)

**MP3** (alternatively also via **MBPT(3)**)

**SDQ-MP4** (alternatively also via **SDQ-MBPT(4)**)

**MP4** (alternatively also via **MBPT(4)**)

* III) Configuration-Interaction methods (not recommended)*

**CID** = configuration-interaction doubles

**CISD** = configuration-interaction singles and doubles

* IV) Quadractic Configuration-Interaction methods (not recommended)*

**QCISD**

**QCISD(T)**

* V) Coupled-Cluster methods*

* a) truncation of the cluster operator T*

**CCD** = coupled-cluster doubles

**CCSD** = coupled-cluster singles and doubles

**CCSDT** = coupled-cluster singles, doubles, and triples

**CCSDTQ** = coupled-cluster singles, doubles, triples, and quadruples (not part of the public release)

*b) additional perturbative treatment of higher excitations*

**CCSD+T(CCSD)**

**CCSD(T)**

**CCSDT[Q]** (not part of the public release)

**CCSDT(Q)** (not part of the public release)

*c) additional approximations in the amplitude equations*

**CCSDT-1**

**CCSDT-1b**

**CCSDT-2**

**CCSDT-3**

**CCSDT-4**

within the *CCn hierarchy*, the following models are currently available

**CC2**

**CC3**

*d) linearized coupled-cluster methods*

**LCCD**

**LCCSD**

*e) unitary coupled-cluster methods*

**UCC(4)**

*f) Bruecker coupled-cluster methods*

**B-CCD** (currently only available via CALC=CCD and BRUECKNER=ON)

* VI) CC and CI calculations with higher excitations*

Via the interface to the *MRCC* code by M. Kállay, calculations at the
following levels are also possible

**CI(n)** with n as the corresponding excitation level

**FCI**

**CCSDTQ**

**CCSDTQP**

**CCSDTQPH**

**CC(n)** with n as the corresponding excitation level

**CCSDT(Q)**

**CCSDT(Q)_A**

**CCSDT(Q)_B**

**CC4**

**CCSDTQ-1**

**CCSDTQ-1a**

**CCSDTQ-1b**

**CCSDTQ-3**

**CC(n-1)[n]**

**CC(n-1)(n)**

**CC(n-1)(n)_A**

**CC(n-1)(n)_B**

**CC(n-1)(n)_L**

**CCn**

**CC(n)-1**

**CC(n)-1a**

**CC(n)-1b**

**CC(n)-3**

The excitation level needs to be here specified for CI(n), CC(n), CC(n-1)(n), etc.
via

**EXCITATION=n**

* VII) State-specific Multi-reference CC calculations using Mukherjee's ansatz*

State-specific MRCC calculations are possible within the **CCSD** approximation and invoked via the
keyword **MRCC=MK**. The used scheme is based on Mukherjee's suggestion for the sufficiency conditions.

**Additional topics in the specification of the quantum chemical method**

*Choice of Reference Functions* (see reference function)

*Open-shell CC treatments* (see open-shell CC treatment)

*Frozen-core treatment* (see frozen-core calculations)

*Partial AO algorithms* (see Partial AO algorithms)

* Available CC modules* (see CC modules)

**Examples**

*closed-shell energy calculations*

*open-shell energy calculations using a UHF reference*

*open-shell energy calculations using a ROHF reference*

*Brueckner-CC energy calculations*

*open-shell energy calculations using PSA- or SR-CC methods*

*unitary CC energy calculations*

*two-configurational SCF energy calculations*

**Recommendation**

Recommended methods for single-point energy calculations are **HF**, **MP2**, **CCSD**, and **CCSD(T)** with in
particular the latter providing high-accuracy results if used with appropriate basis sets.

For open-shell molecules, spin-orbital CC approaches based on both UHF- and ROHF reference functions are recommended with a
slight preference for ROHF reference functions.