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

[Mikael Johansson]

Hello Gerald and All!

On Wed, 12 Mar 2008, Gerald Knizia 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:
>
>   gthresh,twoint=1.e-15
>   gthresh,oneint=1.e-15
>
> 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.

Just a quick note to confirm that this indeed solved my problem; the CCSD 
converged withing 2-5 iterations, depending on the conformation. And it 
was apparently the diffuseness of the basis set (aug-cc-pVQZ) that was the 
root cause, also the symmetry was relatively high, D2/D2h. Thanks!

Have a good weekend,
     Mikael J.
     http://www.iki.fi/~mpjohans