TURBOMOLE manual
v8.0

12.1 Selection Thresholds used in the pnoccsd Program

The accuracy and the computational costs for calculations with the pnoccsd program depend crucially on a number of selection thresholds. Their default values are coupled to the PNO truncation threshold tolpno, such that the errors from all other approximations are smaller then the PNO truncation error. Depending on the application it might be that, in particular for energy differences, certain errors cancel out to a large extent and significant computational saving might be obtained by loosening specific thresholds.

The following thresholds can be set in the data group $pnoccsd:

  • The PNO truncation threshold. The default value is \(10^{-7}\) and gives typically errors in the (total) correlation energy \(<1\%\) which decrease as a rule of thumb approximately with \(\sqrt{\texttt{tolpno}}\).

  • The OSV truncation threshold (cmp. Ref. [24]) which is by default set to \(< 0.1 \times \texttt{tolpno}\). This threshold mainly influences the time for the PNO-MP2 step.

  • The threshold for selecting the local RI basis (cmp. Ref. [24]), which is by default set to \(10^{7/12} \times \sqrt{\texttt{tolpno}}\). This threshold is important for the performance and accuracy of the integral evaluation.

  • The energy threshold for selecting the significant LMO pairs. If not specified in $pnoccsd it is set to \((0.1 \times \texttt{tolpno})^{2/3}\).

  • The threshold for the TNO truncation. The default is \(\texttt{toltno} = \texttt{tolpno}\). Since the TNOs are restricted through the PNOs this is the tightest useful value. Setting it tighter than tolpno will usually not improve the accuracy. It only affects the triples corrections and is important for their costs and accuracy.

  • The selection threshold for LMO triples included for the (T0) and (T) corrections. By default toltrip is set to toltno. It only affects the triples corrections and is important for their costs and accuracy.

  • The selection threshold for PAO domains for OSVs (cmp. Ref. [297]).

  • The selection threshold for PAO domains for PNOs (cmp. Ref. [297]).

  • The selection threshold for PAO domains for TNOs.

For further options and thresholds see Sec. 25.2.25.