The options to define the orbital and the auxiliary basis sets,
the maximum amount allocatable core memory (`$maxcor`), and
the frozen-core approximation (`$maxcor`) have been mentioned above
and described in the previous chapters on MP2 and CC2 calculations.
Apart from this, CCSD and CCSD(T) calculations require very little
additional input.

Relevant are in particular some options in the `$ricc2` data group:

$ricc2 ccsd ccsd(t) conv=7 oconv=6 mxdiis=10 maxiter=25The options

`ccsd`

and `ccsd(t)`

request, respectively,
CCSD and CCSD(T) calculations. Since CCSD(T) requires the cluster
amplitudes from a converged CCSD calculation, the option `ccsd(t)`

is implies the `ccsd`

option.
The number given for `mxdiis`

defines the maximum number of
vectors included in the DIIS procedure for the solution of the
cluster equations. As mentioned above, it has some impact on the
amount of disc space used by a CCSD calculation. Unless
disc space becomes a bottleneck, it is not recommended
to change the default value.

With `maxiter`

one defines the maximum number of iterations for the
solution of the cluster equations. If convergence is not reached within this
limit, the calculation is stopped.
Usually 25 iterations should be sufficient for convergence.
Only in difficult cases with strong correlation effects more iterations
are needed.
It is recommended to increase this limit only if the reason for the
strong correlation effects is known. (Since one reason could also
be an input error as e.g. unreasonable geometries or orbital occupations
as a wrong basis set assignment.)

The two parameters `conv`

and `oconv`

define the convergence
thresholds for the iterative solution of the cluster equations. Convergence is
assumed if the change in the energy (with respect to the previous iteration)
has is smaller than
10^{-conv} and the euclidian norm of the
residual (the so-called vector function) is smaller
than
10^{-oconv}.
If `conv`

is not given in the data group `$ricc2` the threshold
for changes in the energy is set to value given in `$denconv`
(by default 10^{-7}).
If `oconv`

is not given in the data group
`$ricc2` the threshold for the residual norm is by default set to
10 times the threshold changes in the energy.
With the default settings for these thresholds, the energy will thus be
converged until changes drop below 10^{-7} Hartree, which typically ensures
an accuracy of about
1 *μ*H. These setting are thus
rather tight and conservative even for the calculation of highly accurate
reaction energies.
If for your application larger uncertainites for the energy
are tolerable, it is recommended to use less tight thresholds,
e.g. `conv=6`

or `conv=5`

for an accuracy of, respectively,
at least 0.01 mH (0.03 kJ/mol) or 0.1 mH (0.3 kJ/mol).
The settings for `conv`

(and `oconv`

) have not only an
impact on the number of iterations for the solution of the cluster
equations, but as they determine the thresholds for integral screening
also to some extend on the costs for the individual iterations.