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 μH. 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.