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Quantum Chemical Method Input

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) Møller-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.