These method keywords request coupled cluster calculations [Bartlett78], using double substitutions from the Hartree-Fock determinant for CCD [Pople78], or both single and double substitutions for CCSD [Cizek69, Purvis82, Scuseria88, Scuseria89]. CC and QCID are synonyms for CCD. RO may be combined with CCSD for a restricted open-shell energy calculation [Watts93].

#### FC

All frozen core options are available with this keyword; a frozen core calculation is the default. See the discussion of the FC options for full information.

#### T

Include triple excitations non-iteratively [Purvis82, Pople87] (CCSD only). CCSD-T is a synonym for CCSD(T).

#### E4T

Used with the T option to request inclusion of triple excitations for both the complete MP4 and to form CCSD(T).

#### T1Diag

Computes the T1 diagnostic of T. J. Lee and coworkers [Lee89] (CCSD only).

#### Conver=N

Sets the convergence calculations to 10^{-N} on the energy and 10^{-(N-2)} on the wavefunction. The default is N=7 for single points and N=8 for gradients.

#### MaxCyc=N

Specifies the maximum number of cycles for CCSD calculations.

#### TWInCore

Whether to store amplitudes and products in memory during higher-order post-SCF calculations. The default is to store these if possible, but to run off disk if memory is insufficient. TWInCore causes the program to terminate if these can not be held in memory, while NoTWInCore prohibits in-memory storage.

#### SaveAmplitudes

Saves the converged amplitudes in the checkpoint file for use in a subsequent calculation (e.g., using a larger basis set). Using this option results in a very large checkpoint file, but also may significantly speed up later calculations.

#### ReadAmplitudes

Reads the converged amplitudes from the checkpoint file (if present). Note that the new calculation can use a different basis set, method (if applicable), etc. than the original one.

Analytic energies and gradients for CCD and CCSD, numerical gradients for CCSD(T), and numerical frequencies for all methods.

The restricted open-shell (RO) method is available for CCSD and CCSD(T) energy calculations.

The Coupled Cluster energy appears in the output as follows following the final correlation iteration. The CCSD energy is given in the first line below, and the final line reports the energy with triples included:

Wavefunction amplitudes converged. E(Corr)= -75.001924366
…
CCSD(T)= -0.75002048348D+02

The CCSD energy is labeled E(CORR), and the energy including the non-iterative triples contribution is given in the final line.