[molpro-user] Integral thresholds for CCSD(T)

Gerald Knizia knizia at theochem.uni-stuttgart.de
Wed Mar 12 12:43:11 GMT 2008

On Wednesday 12 March 2008 11:51, you wrote:
> I'm having trouble converging the CCSD in a CCSD(T) calculation. I
> managed to converge the CCSD doing a direct calculation, and
> tightening the thresholds using:
> {ccsd
> direct, THR_DTRAF=1.D-12, THR_DKEXT=1.D-12
> start,4000.2
> save,4000.2}

A way to accomplish this in the non-direct CCSD is to set:


The default for twoint (1e-12) is sometimes too high when tightly packed 
molecules with high symmetric are treated with aug-cc-pV5Z and up, and then 
CCSD will refuse to converge. The above statements tend to fix this.

> The VAR(P) doesn't go below ~0.5D-08. So the question is, is there
> anything to tune to try to get the CCSD converged? 

The above should usually work. If not, specifying something like


(i.e., increasing the DIIS dimension) might help. If molecules converge well 
in smaller bases, level shifts of 0.0 might lead to faster convergence.

> If not, is the (T) evaluation expected to give reliable results if I just
> loosen the  convergence threshold of the CCSD part?

This is hard to predict. Total energies are probably reasonable, but you need 
to be very careful with basis set extrapolation procedures.
Gerald Knizia

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