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Re: Question on various recommended thresholds for SA-MCSCF gradient calculations

Thomas Pijper
thomaspijper@hotmail.com


Hello,

There are indeed many parameters that affect MCSCF precision. In addition to the ones you mention, $MCSCF ACURCY as well as options of $MOORTH can be an important as well. Which settings are optimal differs from case to case.

In the case of SA-MCSCF gradient calculations, leaving these parameters at their defaults may lead to inaccuracies in the calculated gradients. (Firefly calculates gradients for SA-MCSCF orbitals through a semi-numerical approach; therefore, small inaccuracies in the calculated energies will have a relatively large impact on the accuracy of the gradient.) This in turn will cause problems like problematic convergence in geometry optimizations.

Whenever increasing accuracy, I always start with the INTTYP and ICUT keywords. ITOL can be increased as well, but typically has little effect. ICUT=11 is often enough, but you can increase further to 13 or even beyond. For SA-MCSCF, I would also always tighten the $MCSCF ACURCY parameter to at least 5.0D-07. In addition, I would also tighten the various options of the $MOORTH group (important for SA-MCSCF gradients!); see the manual for examples.

When you find that a higher degree of precision is needed, I would start with tightening $TRANS CUTTRF as well as either $DET CVGTOL (for ALDET code) or $GUGEM CUTOFF, $GUGDIA CVGTOL, $GUGDM2 CUTOFF (for GUGA code).

This is only a general approach, so you may need find out what works best with your system.

How adjusting these parameters affects performance is difficult to say as this also depends on your system. In general, CVGTOL has little impact (at least for my systems) as the orbital optimization often takes much more time than the preceding CI coefficient optimization. Apart from that, there is little advice I can give you here.

Hope this helps!


Kind regards,
Thom



On Fri Apr 15 '16 8:26pm, GrEv wrote
------------------------------------
>Hello,

>In the case of SA-MCSCF gradient calculations, there are various recommended more tight thresholds in the manual (p. 144), namely INTTYP, ICUT, ITOL, CUTTRF, CUTOFF and CVGTOL. I wonder:
>1) Which of them are most crucial?
>2) How they would affect the computation time?
>3) What should be expected if they are left by default?

>I noticed some inconsistency in the manual, p. 179, input example 3, concerning the above thresholds. In the example, $GUGEM CUTOFF=1D-12, $GUGDM2 CUTOFF=1D-12, while the recommended values are 1.0D-20 and 1.0D-15, respectively. Are these thresholds system dependent?

>Many thanks!
>

[ This message was edited on Sun Apr 17 '16 at 11:59am by the author ]


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